How to test relays using telnet or openssl s_client?

Manual SMTP testing validates relay behavior:

Basic telnet test (no TLS):

telnet mail.example.com 25. Observe greeting. Type EHLO test.com. Then MAIL FROM:<test@test.com>. Then RCPT TO:<recipient@example.com>. Observe each response.

TLS test with openssl:

openssl s_client -connect mail.example.com:465 (implicit TLS). Or openssl s_client -starttls smtp -connect mail.example.com:25. Then proceed with SMTP commands.

What to observe:

Server greeting and capabilities. Response codes at each step. Error messages for failures. Where rejection occurs.

Common tests:

Can you connect? Does STARTTLS work? Will it relay for your domain? What error for invalid recipient?

Manual testing shows exactly what the server does. No abstraction layer; pure SMTP conversation.