Outlook 2003 sending problem

We have ten computers in our office all are using the same gateway to access
the internet. One computer which uses Outlook 2003 periodically has a problem
were we are told that it cannot access the outgoing server (which is provided
by out internet provider) even though that machine has internet access at the
time. The only solution that we have been able to think of is to re-boot the
computer and retype the message in a new email as the original email that was
blocked from going won't go even after the re-boot. Any suggestions that can
help us eliminate this problem would be greatly appreciated.