Cannot send message using the server smtp.mac.com

Hello,

I think this started last night (or very early this
morning, my brain is fuzzy). I've been travelling
about and somehow recently whenever I try sending an
e-mail from my .mac account through the Mail
application, I get a message that says:

"Cannot send message using the server smtp.mac.com

The server response wax: 5.5.1 Invalid user / domaine
name.

Use the pop-up menu below to try a different outgoing
mail server. All messages will use this server until
you quit Mail or change your network settings." . .
.

So far I've tried deleting the account in Mail and
re-adding it, adding "@mac.com" to my username,
re-entering the password to be sure it's correct,
re-starting Mail, restarting my computer, and
changing the port setting.

I don't think this is a problem with my ISP (Comcast,
I'm in Seattle), because this started when I was in
Calgary using my hotel's wifi, then at the airport in
Vancouver using some other wifi &c.). It just started
randomly. One day it worked, the next day it didn't.

I AM able to, say, send it through smtp.gmail.com,
but I'm wary about that because I'm worried that some
e-mail providers will see that the "from" e-mail
address doesn't match the server and will mark it as
spam (I'm not 100% sure of this, but I think that can
happen and I don't want to risk it). Sending from the
webmail works just fine of course, but I don't like
doing that.

Is anybody else having this problem, and does anybody
have a solution? Thanks.