Understanding the Two Different Types of Email APIs
Before diving into integration, it’s important to clarify that the term Email API covers two distinct markets with very different intentions:
Transactional & Marketing Email APIs
(e.g., Brevo, SendGrid, Mailgun)
These Email APIs are designed for notifications, newsletters, and marketing automation. They are often used by companies that need to send high volumes of outbound emails such as password resets, order confirmations, or marketing campaigns.
They focus on deliverability, templates, campaign management, and analytics, not direct access to users’ personal inboxes.
Contextual or On-Behalf Email APIs
(Gmail, Outlook, IMAP)
The second category, and the one Unipile focuses on, is the On-Behalf Email API market. These APIs allow developers to access and act on behalf of a user: reading, sending, and syncing messages from their actual inbox. This use case powers tools like CRMs, recruiting platforms, and sales automation software that need real-time, two-way sync with user accounts.
Why This Distinction Matters
If your goal is marketing automation or transactional messaging, solutions like Brevo or SendGrid are great fits. But if you want to integrate personal or professional inboxes for use cases like sales outreach, recruiting, or support, you need an On-Behalf Email API.
This guide focuses entirely on the latter category, APIs that connect to Gmail, Outlook, and IMAP to send and receive emails directly from users’ accounts in real time.
What Is Email API Integration?
An Email API (Application Programming Interface) lets applications send, receive, and manage email messages through code. Instead of using traditional protocols like SMTP, IMAP, or POP3, it provides a modern RESTful layer to interact with email data using JSON.
It abstracts away complex server configurations, giving developers unified access to multiple email providers. For example:
GET https://api.unipile.com/v1/email/messages
Authorization: Bearer access_token
This single API call retrieves all messages for a connected user, whether they use Gmail, Outlook, or a custom IMAP inbox. In short: an Email API transforms email handling from a server-heavy process into a lightweight, developer-friendly workflow.
How Does an Email API Work?
An Email API acts as a bridge between your application and email service providers. It uses secure OAuth authentication to grant your app access to a user’s inbox and messages.
Typical workflow:
- User connects their account via OAuth (e.g., Gmail or Outlook).
- Your application receives an access token from the provider.
- API calls (GET, POST, DELETE) allow sending, fetching, or deleting emails.
- Webhooks notify your app in real time when new emails arrive or messages are read.
For example, when a new message is received, Unipile sends a webhook event:
{
"event": "email.received",
"account_id": "12345",
"message_id": "abc-6789"
}
This allows your system to update instantly, no polling required.
Related reading: Retrieve Emails with Unipile API
Why Developers Prefer Using Email APIs
Developers increasingly rely on Email APIs for simplicity, reliability, and flexibility. Beyond basic functionality, they deliver significant advantages across security, scalability, and automation. Below are the core reasons developers choose to integrate them.
Fast Integration Across Providers
Modern Email APIs simplify multi-provider connections. Instead of managing multiple protocols (SMTP, IMAP) or vendor-specific SDKs, a single unified API grants access to Gmail, Outlook, and IMAP accounts. This saves weeks of development time and ensures consistent performance across inboxes.
Deliverability and Reliability
High-performing Email APIs manage queues, retries, and reputation to ensure messages reach inboxes rather than spam folders. This reliability is key for maintaining user trust and message consistency, especially for sales and recruiting platforms that rely on timely communication.
Security and Compliance
Security and Compliance are paramount in today’s digital landscape, and Email API integration provides robust security features to protect sensitive data. Encryption, authentication, and adherence to privacy regulations like GDPR are standard with modern Email APIs, giving developers and users peace of mind.
Email API & Unified Communication Feature Overview
Deliver seamless multi-channel communication with Unipile’s unified API. This table highlights all the supported features across major providers, from LinkedIn messaging to Gmail, Outlook, IMAP, and WhatsApp, enabling developers to build rich, real-time inbox and outreach experiences. Whether you’re integrating a CRM, ATS, or any outreach tool, Unipile centralizes every interaction in one secure, developer-friendly environment.
| Aspect | What it means | Why it matters (Dev / Product) | Key metrics / notes |
|---|---|---|---|
| Fast Integration (Multi-provider) | Single unified API for Gmail, Outlook, IMAP instead of multiple protocols/SDKs. | Faster time-to-ship, less maintenance, consistent behavior across inboxes. | TTV ↓, integration steps ↓, code surface ↓ |
| Secure Authentication | Delegated access with short-lived tokens and refresh flow; no password handling. | Compliance-friendly, safer scopes, simpler offboarding/rotations. | Token TTL, scope granularity, secrets in vault/KMS |
| Real-Time Sync (Webhooks) | Event callbacks on received, read, replied, bounced messages. | Instant UX (no polling), accurate timelines for CRM/ATS workflows. | Webhook latency, retry policy, idempotency keys |
| Deliverability & Reliability (Tracking) | Managed queues, retries, reputation; authentication via SPF/DKIM/DMARC with real-time tracking insights. | Ensures messages land in inbox (not spam) and provides detailed analytics on delivery and engagement. | Inbox placement %, open/click rate, bounce rate, retry/backoff |
| Scalability (Large deployments) | Cloud architecture handles thousands of connected accounts and high throughput. | Predictable performance during spikes; lower ops overhead. | Throughput msgs/sec, concurrency, rate-limit headroom |
| JSON Format & Automation | Normalized JSON for threads, labels, attachments, metadata. | Easier parsing, analytics, and AI/automation pipelines. | Parse time, schema coverage, error rate on transforms |
Explore more in What Are the Benefits of Email API Integration for Developers?
Core Features of a Modern Email API
A complete Email API includes all essential email functionalities. Each feature plays a specific role in enabling reliable, secure, and scalable communication.
Send Messages
Email APIs make it easy to send emails programmatically, whether transactional or personalized. Developers can build campaigns, trigger sequences based on events, or automate transactional notifications with precision.
Receive & Parse Inboxes
Access incoming messages with full visibility into subjects, senders, bodies, and attachments. Parsing inbox data enables contextual workflows, such as ticket classification or AI-driven sentiment analysis.
Thread Management
Manage conversation threads efficiently. Retrieve and reply to existing threads to maintain context, improving productivity for CRM, sales, or recruiting tools.
Tracking
Measure performance with delivery, open, and click rates. Tracking endpoints provide analytics for engagement insights and deliverability improvements.
Contact & Calendar Sync
Integrate communication with scheduling. Sync contacts, events, and meetings to deliver a unified user experience inside your application.
Every connected email account (Gmail, Outlook, or IMAP) automatically includes access to the corresponding calendar features, allowing you to create, update, and manage events directly through the calendar API.
Send an email (POST /api/v1/emails)
Minimal multipart example using your DSN and API key:
curl --request POST \
--url https://{YOUR_DSN}/api/v1/emails \
--header 'Content-Type: multipart/form-data' \
--header 'X-API-KEY: {YOUR_ACCESS_TOKEN}' \
--form account_id=YOUR_ACCOUNT_ID \
--form 'subject=Hello from Unipile' \
--form 'body=Hello, this is a test email from Unipile' \
--form 'to=[{"display_name":"John Doe","identifier":"[email protected]"}]'
Tip: You can also use JSON payloads or the official SDKs.
Send attachments
Add files via multipart form-data. Multiple files are supported.
--form 'attachments=@/path/to/file.pdf' \
--form 'attachments=@/path/to/image.png'
Custom headers
Pass extra headers using an array of objects with the X- syntax.
--form 'custom_headers=[{"name":"X-My-Custom-Header","value":"Example value"}]'
Reply to an email
Use the original email’s provider_id in reply_to.
--form 'reply_to=PROVIDER_MESSAGE_ID'
Personalize “from”
Override display name and/or an allowed alias configured on the provider.
--form 'from={"display_name":"Jake Doe","identifier":"[email protected]"}'
Tracking options
Enable open and link tracking to receive real-time events via webhooks.
--form 'tracking_options={
"opens": true,
"links": true,
"label": "myownid"
}'
- Open & click events are delivered to your tracking endpoint.
- For sent/received email events, see webhooks.
Send a plain text email
Use application/json with a text-only body.
{
"account_id": "YOUR_ACCOUNT_ID",
"subject": "Hello from JSON",
"body": "Hello, this is a plain text email",
"to": [{"display_name":"John Doe","identifier":"[email protected]"}]
}
Main Email API Providers You Can Integrate
Before starting your integration, it’s important to understand the main email providers available for connecting user inboxes.
Each provider has its own authentication flow, permission model, and setup requirements. Below are the three core options supported by Unipile.
Gmail API
The Gmail API allows developers to access and manage Gmail mailboxes securely using OAuth authentication.
With Unipile, you connect users’ Gmail or Google Workspace accounts without handling credentials or maintaining mail servers.
OAuth Configuration Steps (via Unipile)
1. Create a Google Cloud project in your [Google Developer Console].
2. Generate an OAuth Client ID (Application type: *Web application*).
3. Add two redirect URIs based on your DSN, one with port and one without port (e.g., `https://your-dsn.com/oauth/callback` and `https://your-dsn:port/oauth/callback`).
4. Enable the Gmail API (and optionally the Calendar API).
5. Invite `start@unipile.com` as a Project Owner so Unipile can verify and activate the OAuth configuration.
Once configured, Unipile handles the secure OAuth exchange and token refresh automatically for all your connected Gmail accounts.
Reference: Google OAuth Integration Guide
Microsoft Outlook API (Microsoft Graph)
The Microsoft Graph API provides access to Outlook mailboxes across Microsoft 365, Outlook.com, and Exchange Online.
It is the standard approach for connecting business email accounts securely through Unipile.
OAuth Configuration Steps (via Unipile)
1. Go to your Microsoft Entra ID (Azure Active Directory) and create a multi-tenant app registration.
2. Add two redirect URIs pointing to your DSN.
3. Under API Permissions, grant: Mail.ReadWrite / Mail.Send / Mail.Read / Calendars.ReadWrite (Optionally)
4. Generate a Client Secret (recommended expiration: Never) and copy its value securely.
5. Configure your credentials inside the Unipile dashboard to complete the connection.
This setup enables your application to send and receive emails on behalf of users across all Microsoft accounts.
Reference: Microsoft OAuth Integration Guide
IMAP (Generic Email Providers)
For mail servers that do not provide native APIs, IMAP remains the universal protocol for mailbox access. Unipile supports secure IMAP connections with automated synchronization and normalization.
Step-by-Step Setup (via Unipile)
1. Collect IMAP and SMTP settings from your provider:
IMAP (SSL/TLS): Port 9932.
SMTP (SSL or STARTTLS): Port 465 or 587
2. Create a new IMAP connection in your Unipile dashboard.
3. Authenticate securely:
- Use AUTH if available, otherwise standard username/password over TLS.
4. Map folders (Inbox, Sent, Drafts, Spam, Trash) for two-way synchronization.
5. Define sync range: choose full history or from a specific date to optimize performance.
6. Set polling frequency: adjust sync intervals to balance freshness and API load.
7. Enable event forwarding in Unipile: new messages or status changes will trigger webhook-like events for your app.
8.Test and monitor errors such as AUTHENTICATIONFAILED or rate limits to ensure stability.
Reference: The Developer’s Guide to IMAP API Solution
Unifying All Providers with Unipile
Instead of managing three different APIs, Unipile lets you connect Gmail, Outlook, and IMAP through a single, unified Email API.
This eliminates the need for multiple OAuth setups, SDKs, or integration layers. With Unipile, your app communicates seamlessly with any inbox while maintaining security, scalability, and real-time synchronization.
Why Choose a Unified Email API?
Managing multiple email integrations can quickly become complex. Each provider (Google, Microsoft, IMAP) comes with its own authentication model, endpoints, rate limits, and error handling. Unipile abstracts all that complexity into one standardized API, so your developers focus on building product features, not managing infrastructure.
Benefits of Unipile’s Unified Approach
One REST API for all inboxes (Gmail, Outlook, IMAP)
→ Access, send, and sync messages across all major providers using the same endpoints.
Centralized OAuth handling for Google and Microsoft
→ Securely manage OAuth tokens, refresh flows, and scopes without needing to build your own identity system.
Consistent JSON response structure
→ Unified data schema for messages, threads, labels, and attachments, regardless of the source provider.
Simplified integration process
→ Connect any account in minutes using a single OAuth flow and API key.
Real-time updates via webhooks
→ Get instant event notifications when new emails arrive, are sent, or read, without complex polling logic.
Scalable for thousands of users
→ Unipile’s cloud architecture handles connection pooling, retries, and queue management automatically.
Top Features of Unipile Email API
Email Features
Account Connection
Hosted Auth: White label email auth
Custom auth: Connect user with credentials
Gmail or Microsoft OAuth: Use your App Oauth screen
Send/Recieve
Send an Email
Reply to an Email
List Emails
Organize
Delete Email
Move Email
List Folders
Get Folder
Webhooks
New Email
Open and Click tracking
CRM & ATS Use Cases Powered by the Email API
Modern CRMs and ATS platforms need on-behalf email integration to keep conversations in sync across Gmail, Outlook, and IMA, plus real-time tracking, unified threads, and calendar coordination. With Unipile’s Email API, your product gets all of that through a single integration.
CRM: Sales Outreach & Account Management
What it enables
-
Unified timeline: sync inbound/outbound emails per contact and deal (threads, attachments, labels).
-
Automated sequences: email follow-ups, multi-step flows, and cross-channel steps (Email → LinkedIn → WhatsApp).
-
Engagement tracking: opens, clicks, replies to prioritize deals and trigger next actions.
-
Contact & calendar sync: book meetings from the thread and track activity inside the deal.
Typical workflows
SDR sequences with reply-stop logic and SLA alerts.
AE handoffs with full context (all emails stay linked to the account/opportunity).
Success check-ins and QBR reminders driven by engagement data.
Key endpoints (examples)
- POST /email/send (on-behalf sending)
- GET /email/threads (conversation context)
- POST /webhooks (new emails & status events)
- GET /tracking/email (opens/clicks/replies)
- Calendar actions available via the calendar API
ATS: Candidate Communication & Hiring Velocity
What it enables
-
Two-way sync with candidates’ email threads across recruiters and hiring managers.
-
Stage-based emails: automatic confirmations, interview invites, and feedback follow-ups.
-
Compliance & auditability: keep sensitive hiring conversations in one secure, SOC 2–certified system.
-
Scheduling built-in: propose slots, create invites, and sync outcomes to the candidate profile.
Typical workflows
Auto-reply on application received → schedule screen → send decision email.
Re-engagement sequences for silver-medalist candidates.
Centralized inbox view for each req, with thread ownership and collision avoidance.
Key endpoints (examples)
- POST /email/send (templated stage emails)
- GET /email/messages (status + attachments)
- POST /webhooks (new message → update candidate stage)
- GET /tracking/email (monitor response latency)
Email Sequences & Automation with Unipile Email API
Automating communication is one of the most powerful use cases of any email API.
With the Unipile Email API, developers can build, schedule, and personalize automated email sequences directly from their software, without depending on external marketing tools. This unified API works seamlessly with Gmail, Outlook, and IMAP, allowing full control over message creation, delivery, tracking, and real-time response handling.
Email Thread Retrieval
Access full email history and real-time sync across all providers.
- Fetch by contact or timeframe
- Parse message content and metadata
- Keep inboxes unified
APIs Used
Cross-Channel: Email + LinkedIn
Start with email, follow up on LinkedIn.
- Email → LinkedIn message
- Stop when a reply is detected
- Thread for both channels
APIs Used
Messaging via LinkedIn & WhatsApp
Send, receive, and manage direct messages across channels.
- Start conversations from workflows
- Handle replies automatically
- Centralize all messages
APIs Used
Google & Outlook Calendar Sync
Build scheduling features into your workflows or bots.
- Create, update, cancel events
- Check availability in real time
- Trigger actions from bookings
APIs Used
Multi-Channel: Email + WhatsApp + LinkedIn
Combine channels in one flow: email → WhatsApp → LinkedIn.
- Event-based triggers
- On-behalf across channels
- Unified KPIs and sequence analytics
APIs Used
Automate personalized, multi-channel follow-ups across Gmail, Outlook, IMAP, LinkedIn, and WhatsApp, ensuring higher reply rates and consistent user engagement without manual effort.
Pricing: Scale Your Email API Integration with Transparent Plans
Unipile offers a flexible pricing model designed for developers scaling their Email API integrations.
Each connected account: Gmail, Outlook, or IMAP: is counted individually and billed according to the total number of linked inboxes your application manages. Pricing automatically decreases as your volume grows, giving you predictable, transparent costs.
For example, connecting one Gmail account and one Outlook inbox counts as two accounts.
-
Up to 10 linked accounts: €49 / $55 per month (base plan).
-
11–50 linked accounts: €5.00 / $5.50 per account per month.
-
51–200 linked accounts: €4.50 / $5.00 per account per month.
-
201–1000 linked accounts: €4.00 / $4.50 per account per month.
-
1001–5000 linked accounts: €3.50 / $4.00 per account per month.
-
5001+ linked accounts: €3.00 / $3.50 per account per month.
If your software connects 15 email inboxes, the total monthly cost would be €75 / $82.50 (15 × €5.00 / $5.50).
Billing runs on a post-paid model, meaning you’re only charged for the active accounts each month. Start free with a 7-day trial, get full access to all email providers (Gmail, Outlook, IMAP), and test your integration before scaling production.
Learn more on our official pricing page: Unipile API Pricing
Manage All Your Email Integrations from the Unipile Dashboard
The Unipile Dashboard gives developers a real-time overview of all connected email accounts in one place. From a single, unified interface, you can monitor authentication health, delivery status, and inbox synchronization across Gmail, Outlook, and IMAP providers.
Each linked account appears with a full operational status, green for active, orange for authentication required, or purple for paused. Developers can reauthenticate, suspend, or remove accounts instantly without managing tokens manually.
Built for scale, the dashboard supports thousands of email connections while ensuring secure OAuth handling, uptime monitoring, and error visibility.
It’s not just an admin view, it’s a control center for your entire Email API infrastructure.
Security First: Unipile Achieves SOC 2 Certification
On October 19, 2025, Unipile officially obtained its SOC 2 Type II certification, validating our ongoing commitment to the highest standards of data security, availability, and confidentiality.
This milestone reinforces trust for developers and businesses integrating sensitive data via Unipile’s Email API. When you connect Gmail, Outlook, or IMAP accounts on behalf of your users, you handle highly confidential communications, messages, attachments, and identity tokens. SOC 2 ensures that every layer of Unipile’s infrastructure protects this information through verified, independently audited processes.
How SOC 2 Strengthens Email API Integrations
-
Secure data handling: All email messages, metadata, and OAuth tokens are encrypted in transit and at rest.
-
Strict access control: Only authorized systems and personnel can access user data, ensuring account isolation for every organization.
-
Continuous monitoring: Infrastructure, API logs, and webhook endpoints are automatically audited to prevent unauthorized activity.
-
Compliance alignment: SOC 2 compliance complements Unipile’s GDPR and ISO27001-based internal standards, ensuring enterprise-grade integration security.
When choosing an Email API provider, this certification guarantees your product inherits trust-by-design. Developers can focus on building email-based workflows and automation without worrying about compliance gaps or data breaches.
Conclusion
FAQs
What is an Email API?
An Email API lets developers send, receive, and manage emails programmatically through RESTful endpoints instead of SMTP or IMAP servers.
What’s the difference between transactional and on-behalf Email APIs?
Transactional APIs (like SendGrid or Brevo) send bulk or automated messages such as invoices or password resets. On-behalf APIs (like Gmail, Outlook, or IMAP via Unipile) allow access to a user’s real inbox for two-way communication.
Is the Gmail API free?
Yes, Gmail API access is free, but limited by quotas and requires OAuth 2.0 user authentication.
Is there any free Email API?
Most providers offer limited free tiers. However, production-grade on-behalf APIs typically charge based on connected accounts or volume.
How can I integrate Gmail, Outlook, and IMAP together?
A unified API such as Unipile connects all three through a single integration, simplifying token management and data normalization.
What are the key benefits of using an Email API?
Faster integration, real-time synchronization, improved deliverability, and reduced infrastructure maintenance compared to custom IMAP/SMTP setups.
Is an Email API secure?
Yes, especially with providers certified under SOC 2 and GDPR. Always use OAuth 2.0 authentication and TLS encryption for data in transit.
How does Unipile differ from Nylas ?
Unipile extends beyond email. It unifies messaging, calendar, and inbox integrations (LinkedIn, WhatsApp, Gmail, Outlook) into one API, offering a broader, real-time communication layer for SaaS developers. See Nylas Alternative.
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