[X-Newbies] mail.app won't send

[Jane]

I had to resort to webmail to send this message............<sigh>

I'm having trouble sending mail again -- smtp server keeps rejecting the address as not allowed within the DNS. This happened before then cleared itself up. It just started at 3 am this morning for no apparent reason.

Ironically, it all started because I tried to post to this list....then I ended up not being able to send elsewhere all night even from a straight Comcast (my cable ISP) address.
I deleted my smtp for my cable and re-added it.

What a pain.. is this mail.apps fault or Comcast's?