Cannot send email using MS Outlook 2003

Getting message "cannot send email, senders address invalid". I am able to connect to internet and receive email, but this ##**$$## Itouch will not allow me to send an email!!!!

I am using Windstream as my isp (mine is actually ctc.net) and the only place I can make complete use of email is in my office with our wireless network.

I am using MS Outlook 2003.

BTW..The geek squade replaced the unit thinking the software had a glitch.

Anyone else having this problem and is there a fix?