can't send email

[Alex Nelson]

Two weeks ago I installed Ubuntu Linux & love it!  Almost all of my 
hardware & gadgets worked right away, & most of those that didn't just 
needed minor tweaking.  Quite an improvement from a year ago when I 
installed Debian & it took me three weeks just to get my graphics card 
working.  The last hurdle is a critical one: I can recieve email via 
Evolution, but I can't send email out(I'm suffering the humiliation of 
sending this email using my wife's Mac).  I've tried changing all the 
different settings to no avail.  The same hardware was used with debian 
running an older version of Evolution all last year with no problem.  
Whenever I try to send email, I get the response:

RCPT TO <xxxxx at xxxxxx.com> failed: Requested action not taken: mailbox 
unavailable

This morning I installed Thunderbird & it was also unable to send email.

I'm sure that it's something simple that I must be overlooking, but I 
don't know where to go from here.  Any suggestions would be greatly 
appreciated.

Thanks!

Brett