Troubleshooting SMTP Issues with Telnet

I have really bad memory. I have to write this down. I just know I will forget it, but need it later.

I had to find out why one user did not receive any mail from us. I tried the usual things, but all looked fine. My colleague suggested to troubleshoot it with telnet. I found good article about troubleshooting SMTP issues. What I did:

• open command prompt
• find server

  >nslookup -querytype=MX [domain]
  ...
  Non-authoritative answer:
  [domain] MX preference = [integer], mail exchanger = [subdomain].[domain]
  ...

• connect to that machine with telnet

  >telnet [subdomain].[domain] 25

• good

  220 SMTP Proxy Server Ready

• not so good

  554 Transaction Failed Listed in connection control deny list