When a refresh token expires or is revoked, a naive implementation throws an error and stops syncing - silently. The user doesn't know their inbox stopped syncing until they notice stale data. Fix: implement a revocation detection layer that catches invalid_grant errors, marks the linked account as requiring re-authorization, and notifies the user via your product's notification system.

Polling every 10 seconds across 200 linked accounts burns through Gmail's per-project quota within hours. Microsoft Graph starts returning 429 Too Many Requests. The result is silent sync failures for all accounts - not just the ones that triggered the throttle. Fix: use webhooks as your primary mechanism with a 5-minute polling fallback, and implement per-account rate limiting with exponential backoff on all 429 responses.

Raw MIME is large, hard to query, and expensive to parse on read. A single email with attachments can be hundreds of kilobytes. Fix: parse the MIME on ingest: extract HTML body, plain text fallback, headers, and attachment metadata separately. Store attachment binaries in object storage (S3 or equivalent), not in your primary database. This alone cuts your storage costs by 60-80% for typical corporate mailboxes.

Gmail's threadId works only within Gmail. If your app shows a thread that spans a Gmail account and an Outlook account (e.g., a reply sent from a different mailbox), the native threading IDs are useless. Fix: build a cross-provider threading engine based on the Message-ID, In-Reply-To, and References headers. Normalize subject lines (strip Re:/Fwd: prefixes) as a fallback for mails missing these headers.

Delta sync relies on a stored cursor: Gmail's historyId, Microsoft Graph's deltaLink, IMAP's MODSEQ. If your sync worker restarts and the cursor is stored only in memory, you lose your position in the change stream. The next sync starts from scratch, duplicating all historical messages or missing the gap. Fix: persist the cursor to your database after every successful sync cycle, atomically with the last batch of processed changes.

For CRM use cases, you need both inbound (received) and outbound (sent) emails to build a full communication timeline. Gmail's INBOX label only covers received mail; you also need SENT. Microsoft Graph requires querying the SentItems folder separately. IMAP requires selecting the Sent folder explicitly. Fix: sync all relevant folders on account setup, not just INBOX, and map provider-specific folder names to normalized types in your data model.