Email clients in Ubuntu

[robfleet at ix.netcom.com]

Greetings all,

I have developed a sudden strange problem that I can't seem to remedy so I thought I'd throw it out for comment...

I have two desktop machines running Ubuntu (9.04) in my home.  My machine downstairs is presently awaiting a new motherboard from IBM, hopefully tomorrow, but it has both XT and Ubuntu on it in a dual boot configuration.  My machine upstairs is configured with only Ubuntu 9.04 (no Windows).  Sometime in the past several days, I cannot get any e-mail client to send mail (whether I use Evolution or Thunderbird).  It receives email fine and my machine downstairs (before IBM determined I needed a new motherboard) was able to send mail while in either Windows or while running Ubuntu.  We spent quite a bit of time on the phone with our ISP inquiring as to whether they had made any recent changes to their SMTP server(s) but they claim not.

It is a complete mystery to me as to why my one machine can send mail but my other one cannot.  It does not appears to be client specific since the same symptom appears in Thunderbird as in Evolution.  

If anyone has any suggestions, I'd love to hear 'em.

Many thanks in advance.

Rob