JustfixemailFastmail Not Syncing Across Devices
Fastmail is IMAP-based end to end, so in my experience most sync gaps come down to one specific app falling behind rather than an account-wide problem. The web app itself is very rarely the actual source.
1. Confirm it's one app, not everywhere
Check fastmail.com directly. New mail's there but missing on one device? The problem's local to that device's app, not your account.
2. Check app password status (for third-party IMAP clients)
Fastmail wants app-specific passwords for third-party mail clients — your main login won't work for IMAP/SMTP setup at all.
- Settings → Password & Security → App Passwords.
- Confirm the one your mail client is using still shows as active — a deleted or regenerated one stops working immediately, with no warning.
- Not sure? Revoke the old one, generate a fresh one, update it in your app.
3. Re-sync a lagging folder
In desktop clients like Thunderbird or Outlook, right-click the account or folder and look for a "resync" or "empty local cache and redownload" option. This fixes most local sync database corruption without needing to remove and re-add the whole account.
4. Check folder subscriptions
Because Fastmail is IMAP-based, folders need to be "subscribed" in some clients to show up at all. If an entire folder — not just new mail — is missing on one device, check that app's subscription settings.
5. Mobile app not updating
- Check background app refresh or battery optimization isn't restricting the Fastmail app.
- Manual pull-to-refresh not bringing in new mail? Log out and back in.
FAQ
How do I tell if it's account-wide or just one app?
Check Fastmail's web app directly — if mail is there and current, the account itself is fine and the problem is specific to whichever app isn't syncing.
An entire folder is missing on one device — what causes that?
Check folder subscriptions on that specific device or app — folders can be individually subscribed or unsubscribed per connection, so a missing folder doesn't necessarily mean lost mail.
Could a revoked app password cause silent sync failures?
Yes — if an app password was revoked or expired, that app stops syncing without always giving a clear error message. Confirm the app password is still valid under your account's security settings.