[cla]

I suspect this post has more to do with Mail.app than ssh-tunneling.
Anyway, I set up a tunnel like this:

> ssh -L5525:my-smtp.server.com:25 my-ssh.server.com

I test the tunnel using

> telnet 127.0.0.1 5525

and it connects just fine. The thing is, I can't get Mail.app to connect. I've set 127.0.0.1 as host using port 5525, but Mail.app complains about the server not supporting authentication.

I set the mailserver up myself, so authenticating shouldn't be the problem.

[cla]

Um, I just solved it by changing my-ssh.server.com to another server.

my-ssh.server.com is actually a firewall protecting my-smtp.server.com. I still don't know why I couldn't get the tunnel to work using my-ssh.server.com. Telnetting from there to port 25 on my-smtp.server.com works fine, so there shouldn't be a firewall or internal dns issue...