you mail account may use two separate accounts to receive and send mail - check that your account is set properly - or just reset it may do the trick - also check that your ISP hasn't changed the account names - similar happened to me when I upgraded to XP as the mail accounts differ for the XP version.
If the above doesn't work let me know.
Can you help - we are having problems with outlook express for a while now. Sometimes it seems to send our emails and other times it "just hangs"and will not send. We can press send and receive and nothing happens. Likewise with photographs - we can receive quite a lot, but cannot send, not even one.