Mail error codes, and who is actually at fault
Most guides treat every 5.x code as a dead recipient. Most of them are not. A 535 5.7.8 is your own password, a 451 4.7.1 is a throttle asking you to slow down, and suppressing the recipient in either case blacklists a healthy address. Each page below carries the real provider transcript, the cause, the fix, and the wrong reactions to avoid.
Your mailbox is misconfigured
- 535 5.7.8 — Username and Password not accepted
Gmail, Google Workspace, SMTP
- FAILED_PRECONDITION — Mail service not enabled
Gmail API, Google Workspace
- invalid_grant — OAuth token revoked or expired
Gmail API, Google Workspace, Microsoft 365
- 550 5.7.1 — Message rejected on policy grounds
Microsoft 365, Outlook, Gmail, Yahoo
- 554 5.7.1 — Blocked by spam filter or blocklist
Gmail, Microsoft 365, Yahoo, Proofpoint
You are being rate limited
- 550 5.4.5 — Daily user sending limit exceeded
Gmail, Google Workspace
- 451 4.7.1 — Ratelimit exceeded
Hostinger, Titan, shared SMTP hosts
- 421 4.7.0 — Too many messages - try again later
Gmail, Microsoft 365, Yahoo
The recipient address is the problem
- 550 5.1.1 — Recipient address does not exist
Gmail, Microsoft 365, Outlook, most SMTP servers
- 552 5.2.2 — Mailbox full / over quota
Gmail, Microsoft 365, cPanel, most SMTP servers
Transient infrastructure
- Too many simultaneous connections — IMAP connection limit reached
Gmail IMAP, Microsoft 365, Dovecot, cPanel