Troubleshooting SMTP Issues with Telnet

tags: code

Estimated reading time is 1 minute.

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

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