Thanks Bill for the response!
I figured it out essentially
Actually the domain was configured as a sub domain since I was trying to get around a DNS'ing issue of having domain.com going to my www server and thought that if I created a sub domain things would be easier.......
At the beginning all was fine but then I realized (I was still learning DNS at the time) that if I wanted mail sent to the proper domain things would have to be done properly.
Anyhow, it turns out while checking the email source that everything is ok so it seems like a bug with Thunderbird in Fedora 15 x64.... Fedora F16 x64 T'bird works fine and additionally Alpine too.
--so sorted basically.
Now quick question.
When I finally do come to upgrade, needs resources on my behalf as current mail server is PIV with 1.5GB RAM;
if I do a mailbox dump and then import those into the new machine... as long as the accounts are the same, ie. same username etc....
do the hostname + FQDN need to be the same??
As in if I name the system: haven.domain.com in the /etc/hosts file, but point the DNS to domain.com eliminating the need for aliases so that if I was desperate for aliases I could just add the haven.domain.com alias.
Release 5.0.16_GA_2921.RHEL5_20090429052155 CentOS5 FOSS edition