Troubleshoot custom SMTP

Connection refused / timeout

• Wrong host or port.
• Firewall blocks outbound 587 from EonMailer infrastructure — confirm with support if self-hosted relay whitelists IPs.

Certificate errors

• Hostname mismatch on TLS cert — use hostname that matches cert CN/SAN.
• Try STARTTLS on 587 vs SSL on 465 per IT spec.

Authentication failed

• App password required (Microsoft/Google) — regular password won’t work.
• Username must be full email vs local part — ask IT which format.

Relay access denied

• Sender address not allowed for authenticated user.
• IP not on relay allowlist — provide EonMailer egress IPs if IT requires.

Rate limiting

• 421 or 452 responses — reduce send rate in campaign throttling settings.
• Split broadcast across hours.

Messages queued but not delivered

• Check relay logs on client side.
• Recipient server greylisting — retry normal for cold IPs.

No bounce feedback

• SMTP may not push webhooks — import bounces manually or migrate to API provider for ops visibility.