Question : How to Check Your SMTP Connection

If your emails aren't going out, the first thing to test is the SMTP connection itself. A 30-second check tells you whether the server is reachable, whether TLS is up, and whether your credentials work.

Quick test with telnet (port 587, no TLS)

1. Open a terminal.
2. Run: telnet your-smtp-host 587
3. You should see a 220 greeting from the server.
4. Type EHLO yourdomain.com — the server should reply with 250 capabilities.
5. Type QUIT to close.

Test TLS with openssl (port 587 + STARTTLS)

1. Run: openssl s_client -starttls smtp -crlf -connect your-smtp-host:587
2. Check the certificate chain and verify return code 0 (ok).
3. Type EHLO yourdomain.com to confirm authentication is offered.

Test full sending with swaks

Swaks is the fastest way to send a real test email:

• swaks --to [email protected] --server smtp.host:587 --tls --auth LOGIN --auth-user you --auth-password ••••
• Watch every step (greeting, EHLO, STARTTLS, AUTH, MAIL FROM, RCPT TO, DATA, .).

Common SMTP errors and what they mean

• 421 — service not available, often rate-limit or IP block.
• 535 — authentication failed, check user/password.
• 550 — mailbox unavailable, recipient rejected.
• Connection refused — firewall blocks the port or wrong host.
• TLS handshake failed — incompatible TLS version or expired certificate.

Related reading:

• SPF configuration
• DKIM configuration
• DMARC record
• What is deliverability?