Get this message when trying to send e-mails via outlook express: 'couldnt
send message because the server didnt accept the address.'

I know the address is correct and it doesnt seem to be a problem with my
internet operator (have tried sending e-mails via Mozilla Thunderbird and it
works) so why do I get this message??

Have recently uninstalled a trial of office 2007 and installed office xp pro
(2002). This is the first time I have tried outlook express on this computer.
On my old computer there were no problems. Can there be issues caused by
uninstalling the office 2007 trial?