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