I am subscribed to a discussion list as "xxx@sxu.edu" but it won't let me post because my return address is "xxx@mail.sxu.edu". How do I change my return address using the Web interface?
From your main menu scroll down to Options and click on Change User Information.

Scroll down to the box labeled ReplyTo and replace "xxx@mail.sxu.edu" with "xxx@sxu.edu". Then click Save. (Please note this only affects messages you send from the IMail Web interface. If you use other e-mail client programs such as Outlook or Netscape Messenger they have their own settings for your return address.)

Your return address is displayed every time you Compose a message in the "From" line at the very top.

Many discussion lists only allow you to post to the list if the address you are sending from matches the address you used when you subscribed to the list. So, you want to be consistent about which form of your e-mail address that you use, "@sxu.edu" or "@mail.sxu.edu". This is especially true if you use more than one e-mail client; for example Outlook or Netscape Messenger on campus, but the IMail Web interface from home.
Alternatively, if you do not want to change your reply-to address you could resubscribe to your discussion list with the preferred form of your address.