Fastmail Login and Sending Problems
Quick answer: Generate a fresh app password under Password & Security rather than using your main password in third-party apps, and confirm SMTP settings if sending fails.
Devices not staying in sync, but login and sending both work fine? See our guide on Fastmail not syncing across devices instead.
1. Can't sign in on the website
- Use password recovery rather than retrying repeatedly, which can trigger a temporary lock.
- Using a custom domain with Fastmail? Confirm you're signing in with your actual Fastmail username, not just the custom domain address — these can differ, and it trips people up.
2. Third-party app rejects the password
Almost always an app-password issue, not a wrong main password.
- Settings → Password & Security → App Passwords.
- Generate a fresh one for the app giving you trouble, and use that instead of your main Fastmail password.
- If an old app password got revoked — intentionally or as part of a security review — any app still using it fails silently rather than showing a clear "wrong password" message. This one catches people off guard constantly.
3. Outgoing mail stuck or rejected
- Confirm SMTP settings:
smtp.fastmail.com, port 465 or 587, authentication required, using an app password. - Check for an oversized attachment on the stuck message.
- Sending "as" a custom domain? Confirm that domain's SPF/DKIM records are actually set up correctly in DNS — Fastmail's domain setup pages show the exact values.
4. Two-step verification issues
Using an authenticator app? Confirm your device's clock is properly synced — a drifted clock rejects otherwise-correct codes.
Check status.fastmail.com if login or sending is failing for everyone at once, not just you.