0x800ccc79 outllook express domain missing or malformed 550

when I send email out this is the error. Also if I try to forward. It works with web mail but not with outlook express and Thunderbird.
solutions I tried (using windows xp)
port change from 25 to 587
my server requires authentication use same setting as incoming
disabled anti virus avg
disabled firewall
scanned for spy ware with spy bot ad aware
recreated email account. None of this worked.
Even did a system restore to when it was working without a hitch

then yesterday when on the phone with comcast tech. It worked. now this morning not working again. I am willing to try anything at this point. Any suggestions