Should I see email on port 25 with netcat?

Question

Assuming I have a domain forwarded properly to a box (ie, I can see a webpage on the box from the outside world) If I use netcat to listen on some ports from a ssh session:

nc -l 587

nc -l 25

and send mail to the server user@mydomain.com

I should see the beginning of the handshake in the send attempts? I know my ISP does not block port 25 because I can see a browser header with netcat if I go to http://mydomain.com:25

Answer 1

Tons of things could be going wrong:

1) SMTP requires the server talk first, so the email sender is probably waiting for your server to say something.

2) If you're not running a valid mail server, email senders may mark your host as unresponsive and backoff for several hours before retrying.

3) If you just changed your DNS, it may take hours/days for that to propagate. (Not everyone respects TTL, especially low ones.)

4) Some servers are configured to drop mail if the source or destination SPF isn't set up properly. (To encourage everyone to use SPF.) Try adding a SPF record. (And make sure your MX record is correct.)