// how to fix
Not sure what is wrong?How to fix email problems
Your test or scan found a problem? Find the matching guide below: what it means, why it matters, and how to fix it step by step.
Bounce 60
DNS 52
SPF 40
Reputation 39
DMARC 38
DKIM 34
Security 26
Gmail 21
Blacklist 18
Microsoft 365 17
Mailing list 16
Content 15
Mail server 13
Headers 12
Reverse DNS 12
SMTP 12
Spam score 12
Outlook 10
MX 9
Amazon SES 7
Google Workspace 5
TLS 5
Yahoo 5
Mailgun 4
Postmark 3
Apple 2
SendGrid 2
WordPress 2
Zoho 2
ARC 1
BIMI 1
Cloudflare 1
E-commerce 1
GMX 1
Resend 1
T-Online 1
12 guides · tag SMTP
| Problem | In short | Tags | |
|---|---|---|---|
| Amazon SES SMTP "535 Authentication Credentials Invalid": fix it | Amazon SES rejects your SMTP login with 535 Authentication Credentials Invalid. Use real SMTP credentials, the right Region and endpoint, and test them. | Fix it | |
| Amazon SES "Throttling: Maximum sending rate exceeded" (454): fix it | Amazon SES returns "Maximum sending rate exceeded" or "Daily message quota exceeded". Check your quotas, slow down sending and request an increase. | Fix it | |
| Mailgun "401 Forbidden": wrong API key or EU/US region mismatch | Mailgun returns 401 Forbidden or Unauthorized. Check the API key type, the "api" username and whether your domain uses the US or EU endpoint. | Fix it | |
| Gmail SMTP "535-5.7.8 Username and Password not accepted" (and 534-5.7.9) | Gmail SMTP login fails with 535-5.7.8 or 534-5.7.9 "Application-specific password required"? Use an app password or OAuth instead of your password. | Fix it | |
| Google Workspace SMTP relay "550 5.7.0 Mail relay denied": fix | smtp-relay.gmail.com rejects your server with 550 5.7.0 Mail relay denied or Invalid credentials for relay? Register the IP, fix HELO or use SMTP AUTH. | Fix it | |
| Microsoft 365 "535 5.7.139 Authentication unsuccessful": how to fix it | Your app or printer cannot log in to smtp.office365.com. Learn why 535 5.7.139 appears (SMTP AUTH off, security defaults, MFA) and how to fix it. | Fix it | |
| Microsoft 365 "530 5.7.57 Client not authenticated to send mail": fix | Your app, printer or server sent mail to smtp.office365.com without logging in. Fix 530 5.7.57 with SMTP AUTH, direct send or a relay connector. | Fix it | |
| SMTP port 25 blocked: "Connection timed out" on AWS, Azure, GCP, VPS | Your server cannot connect to other mail servers on port 25? Cloud hosts and home ISPs often block it. Test it and relay through port 587. | Fix it | |
| Postfix "553 5.7.1 Sender address rejected: not owned by user": fix | Postfix refuses your From address with 553 5.7.1 not owned by user or not logged in? Fix smtpd_sender_login_maps or send with the right login. | Fix it | |
| SMTP "421 Too many connections" or rate limited: how to fix it | Receiving server answers 421 too many connections or rate limit? Slow down your sending with fewer connections, or adjust Postfix limits. | Fix it | |
| PHPMailer "SMTP Error: Could not authenticate": how to fix it | PHPMailer fails with SMTP Error: Could not authenticate? Turn on debug output, then fix the login, port, encryption or provider settings. | Fix it | |
| Scan to email not working: set up a printer with Microsoft 365 or Gmail | Printer or scanner can no longer send scans by email? Choose the right method for Microsoft 365 or Gmail: SMTP AUTH, direct send, relay, app password. | Fix it |
Not sure what is wrong?
Send a test email or scan your domain: the report tells you which problem you have.