Where is the mail app log?

Is it an error dialog from mail, or an email about the error from mail or the mail server?

Can you send other emails via the same server?

AK

As far as I know mail doesn't log those. It is generally a matter of either connectivity (eg I get it when trying to send a message via a server, but the part of the LAN I am currently using is blocked from accessing that server) or of a valid email address, where the server is able to verify the address because it is local to the server. It may also occur where the both the sender and recipient are outside the domain of the server, but that doesn't sound like your problem.

If you really want to see what is happening, you will need to run a network sniffer (eg ethereal or MacSniffer).

AK

You can always talk directly to the server with telnet! Bring up a terminal window, and try this:

telnet smtp.yourisp.com 25
trying ...
connected to smtp.yourisp.com
escape character is ...
220 smtp.yourisp.com ...
HELO
250 smtp.yourisp.com ...
MAIL FROM: <youremailaddess>
250 sender OK
RCPT TO: <targetemailaddress>
250 recipient OK
QUIT
...

You type the bolded bits. If any of the responses are not 220/250 that is likely your problem.

AK

it's something that almost every other
mail app lets you see.

Well, why don't you try another mail client (e.g. Thunderbird) then, and see whether the problem happens there as well? That surely would help you to find where the problem really is.

Unfortunately, there is no mail client that has all the features we want -- why would you be using Mail otherwise? 🙂

Thus, there are only three things that you can do about this: (1) switch to another mail client that has the features you want most, (2) use whatever mail client you prefer most of the time and use others for specialized tasks the one you prefer doesn't handle, and/or (3) send feedback to Apple at http://www.apple.com/macosx/feedback/.

Anything else is barking up the wrong tree.