When you send an email, there are two pieces of "sender" information... a FROM address and a REPLY-TO address. This is provided so that if you want to send from one account, but want people to reply to another. In this fashion, when they click the "reply" button in their email client, their TO: is populated with whatever the sender specified as the REPLY-TO address (instead of the FROM address).

Note that this is not a server functionality, it's part of the mail client which sends the actual mail... (the mail client being what you use for sending/receiveing mail, like Outlook, or Outlook Express). The mail client simply inserts this REPLY TO information as part of the message info when it sends it.  Here are instructions for finding this REPLY TO value on Outlook or Outlook Express:

Outlook:
- click TOOLS, EMAIL ACCOUNTS
- on the pop-up, select VIEW OR CHANGE EXISTING EMAIL ACCOUNTS, and click NEXT.
- on the Email Accounts page, select the email account (you may have several, select the one we're discussing here) and click CHANGE button.
- on the email settings page for that mail account, click the MORE SETTINGS button
- on the general tab, the REPLY EMAIL address field is where the info is... Note, if you don't want the reply address to be different than the sending address, then you should leave the reply address field empty.

Outlook Express:
- Click TOOLS, ACCOUNTS
- on the pop-up, click the MAIL tab
- on the MAIL tab, select the email account in question, and click PROPERTIES
- the reply address field will be on that page. Note, if you don't want the reply address to be different than the sending address, then you should leave the reply address field empty.