Talking to SMTP / Mail via telnet to diagnoze mail problems on a server

Mailserver diagnoses can be tricky: I personally find the easiest way to do it still using telnet in combination with the server logs to find out what exactly is wrong.

It is very easy to do:

telnet thehost.com 25

helo: myhost.com
mail from: me@myhost.com
rcpt to: someone@thehost.com
data

… type some data …
.

quit

That is all; it will help to exactly determine where it goes wrong.

The case we had today;

– postfix didn’t send mail ; it gave connection refused
– thoughts; firewall, network, dns etc.
– we tried the above and it worked fine, meaning postfix was the one refusing, not the network/firewall etc
– we fixed the postfix config and all was fine

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