SMTP authentication errors when sending

Before you start

Use this page if your app is failing to authenticate to your SMTP provider. If your FormReceipt account-verification email is missing, use account resend and inbox checks first.

Implementation checklist

1. Confirm SMTP host, username, and password or token.
2. Confirm the expected port and encryption mode from your provider docs.
3. Test credentials in the provider dashboard if available.
4. Confirm the sending account is allowed for SMTP submission.
5. Check provider logs for reject reason and exact timestamp.

Port and TLS pairing

• Port 587 usually expects STARTTLS.
• Port 465 usually expects implicit TLS.
• Mismatched port and TLS mode can fail even with correct credentials.

Policy and rate-limit checks

Cloud and hosted environments can hit provider security policies. Review region locks, IP allowlists, suspicious-login protections, and rate limits for your SMTP account.