Sender domain not compliant....

["Allen J. Fulleton" <ajfulleton@mindspring.com>]

I am a newbie and just installed a basic Debian from CD's on my
stand-alone dialup system.  I am attempting to get mail working first.
I installed Fetchmail, Mutt and Smail (recommended by Debian?), removed
Exim,  and I can pickup mail but when I try to send mail to mindspring,
I get:

'Sender domain not compliant with RFC 882 6.2.7' and according to the
logs, the mail is not launched.  I checked info & man pages for the
Smail & config files but I am lost.  Can someone point me to the docs
that explain this?

If you could please reply to me directly since subscribing to the mail
list overwhelmed me because I only check mail once daily.

Thanks in advance,

..Allen