I'm getting errors with sending in Kmail and the error messages have no information:

"Sending failed, message will stay in outbox, following transport used:"

So I'm hoping that there's an error log or something that I can use to help diagnose this. I've tried fiddling with all of the obvious stuff. Is there? Where can I find this?

I don't know, if KMail logs there, but maybe:
~/.xsession-errors

EDIT: Moved to Office & Productivity

I doubt if you are going to find anything more than the error message you already know.

Why not list the settings you give in the sending tab of KMail? Obscure username and passwords if they are used. It's especially important to check whether you need authentication to send.

Maybe have a look at the encryption. KMail supports checking which encryptions the server supports.

Well, the thing is, I'm having this problem on a new computer running SUSE, but had no problem on my old computer running Xubuntu. I think that I've been fairly meticulous in making sure that all of the settings are the same, but there's always a possibility that I've missed something.

I'll check back and post here with the settings - unfortunately, this is a work computer so I can't do it until Monday.

All right, these are the settings. Everything is identical except in Xubuntu I'm using version 1.11.2 and in SUSE I started with version 1.10.(something) but upgraded thinking that that might help with this problem. I'm now using version 1.12.2.

General Tab
Name: xxx
Host: xxx (called Outgoing Mail Server in the SUSE ver.)
Port: 25
Encryption: None

Advanced Tab
everything unchecked, nothing in the Precommand field

It seems to me that there isn't much to work with here, but maybe you know something that I don't? Is this a SUSE issue? Can you think of where I might start looking to resolve it in that case?

Outgoing server is about the only thing that can go wrong there. Are you certain that no encryption is required? Is the server an ISP's? If so, they should have setup instructions on their web pages. It's all to easy to think that everything is the same when there's actually a small difference. For instance, most ISPs seem to use smtp.domain.name, whereas mine uses mailhost.domain.name. If you use gmail then you must give login details to send as well as to receive. This sort of thing you only find by browsing their web pages.

Well... We run our own mail server. We don't have a real admin for it, but I do have access. Obviously, there's always a chance that I've missed something, but there just isn't that much here to miss. My assumption is that there's some difference between SUSE and Xubuntu to blame here, since everything else is the same.

Thanks, I'll keep looking.