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Deliverability Diagnostics & Common Errors

Common SMTP Error Codes

Section titled “Common SMTP Error Codes”

1. 535 Authentication Credentials Invalid

Section titled “1. 535 Authentication Credentials Invalid”
  • Cause: Incorrect SMTP username or password.
  • Fix: For Google Workspace or Microsoft 365, you must generate an App-Specific Password with 2FA enabled. Standard account passwords will be rejected.

2. 550 Relaying Denied / Sender Address Rejected

Section titled “2. 550 Relaying Denied / Sender Address Rejected”
  • Cause: Your From Email Address does not match the authenticated domain or SMTP account.
  • Fix: In Settings → General, ensure the From Email Address domain matches your verified domain in SES, SendGrid, or your SMTP host.

3. Connection Timed Out (Port 25/587/465)

Section titled “3. Connection Timed Out (Port 25/587/465)”
  • Cause: Your web hosting provider (e.g. AWS EC2, DigitalOcean, Linode) blocks outbound mail ports to prevent spam abuse.
  • Fix:
    1. Switch to an API Transport (SES, SendGrid, Resend, Postmark), which operates on standard HTTPS port 443.
    2. Or open a ticket with your host to unblock port 587.

4. RFC 8058 Header Rejected by Mail Client

Section titled “4. RFC 8058 Header Rejected by Mail Client”
  • Cause: Sending marketing broadcasts over unencrypted HTTP.
  • Fix: Ensure your site is served over HTTPS with a valid SSL certificate. MailFuz requires https:// for RFC 8058 token verification.

Deliverability FAQ

Section titled “Deliverability FAQ”

Q: Why do my emails land in Gmail’s “Spam” folder?

Section titled “Q: Why do my emails land in Gmail’s “Spam” folder?”

A: Check that your domain publishes a valid SPF record (v=spf1 ...), a 2048-bit DKIM key, and a DMARC policy. You can verify all three with MailFuz’s DNS Authentication Auditor.

Q: How many emails can I send at once?

Section titled “Q: How many emails can I send at once?”

A: MailFuz itself has no hard limit. The throughput depends on your chosen transport (e.g. Amazon SES default tier is 14 emails/second; Resend is 10/second). For large volumes, offload sending to the MailFuz background queue or WP-CLI runner.