Could be an incorrect account setup for the outgoing mail server or a firewall, for the account setup 1st check what type of account it is hes using ie pop3 or http then in outlook express goto tools>accounts, select the mail tab highlight the account u want to configure and select properties on the right...
In the dialog box that pops up select the server tab and check the outgoing mail section, if u use a pop3 account (the most popular) it should read "smtp.your mail provider" for example my one reads "smtp.ntlworld.com" without the " marks, if u use a http mail like hotmail then it needs to point to the correct url then goto the advanced tab and check what port it is using, outgoing mail (smtp) should be 25 u wont have the tab for http.
If all that is correct check your firewall and make sure the program is allowed to send mail.
