Not always (seems to be in streaks) but often Outlook 2003 refuses to send an email. My ISP tells me the issue is that I am sending before receiving, so my ID is not "on the list." Apparantely this is an anti-spam measure. Outlook seems to have no way to set the order. Right now I have to go through the Tools/Options/Email/Edit and set the account to Read only; force a check (F9), then go back and check 'Send' and do another F9, and it works. I believe following a Read one has a window of time to do a Send before the "approved list" is flushed.
I have posted this before and no response; hopefully not because there is no solution! This is very time consuming just to send one email.