Like many other UNIX systems administrators, I decided to migrate my UNIX mail hub to Exchange Server, because, in my opinion, Exchange is easier to manage. I resolved most questions, except one: How do I emulate the functionality of the UNIX aliases file? The UNIX aliases file is a text file that contains forwarding addresses for specific users. For incoming mail, the UNIX sendmail daemon checks the aliases file to determine whether the daemon needs to forward the incoming message to another messaging server where the user's mail account resides. For example, if I want the UNIX mail hub server (unixmail.fzo.dec.com) to forward my mail for the address email@example.com to another mail server (vmsmail.fzo.dec.com), I add an entry to the end of the aliases file, as Figure 1 shows. If an Exchange Server computer is functioning as the primary Internet mail hub for all mail servers within an organization, you must develop a mechanism for relaying Internet mail for non-Exchange users to their appropriate mail server (e.g., UNIX, OpenVMS, GroupWise, Lotus Notes).
Emulate the Aliases File
Exchange Server doesn't implement an equivalent aliases file, but you can emulate the file's functionality by using a Custom Recipient with two Simple Mail Transfer Protocol (SMTP) addresses. This technique forwards mail to users not local to the Exchange Server mail hub.
From the Exchange Administrator, perform the following steps. Go to File, New Custom Recipient. On the New E-mail Address dialog box, which you see in Screen 1, select Internet Address. On the Internet Address Properties page, which you see in Screen 2, enter the non-Exchange user's final address. In this example, Joe Unix, a non-Exchange user, has requested that Exchange Server relay his mail to a UNIX mail server.
After you enter the Internet address, an empty properties page for the non-Exchange user will come up. At a minimum, complete the First, Last, and Alias fields, according to your organization's naming scheme, as Screen 3 shows. Select the E-Mail Addresses tab. The default SMTP address is the user's destination mail server. If you want the user to receive mail at an address such as firstname.lastname@example.org, you must create a secondary SMTP address. Select New, and in the box on the Internet Address Properties screen, enter the secondary SMTP address in the format appropriate to the local organization (e.g., email@example.com).
The resultant properties page, which you see in Screen 4, shows both the default SMTP address, which is the user's destination mailbox, and the new secondary SMTP address. Exchange Server can now reroute all mail destined for JUnix@compaq.com to the appropriate mail server (e.g., Joe.Unix@AnyUnixServer .compaq.com).
Exchange and UNIX Interoperability
The technique I've described replaces the functionality of the UNIX aliases file for rerouting incoming mail that is destined for firstname.lastname@example.org, where the user's mailbox resides on a UNIX mail server. The UNIX mail server can route its outgoing mail, but when Exchange Server is the incoming SMTP mail hub, Exchange Server won't reroute mail without secondary SMTP address.
This technique has one restriction: Exchange Server has no sendmail rules for rewriting Reply-to addresses if the end user is using a Post Office Protocol 3 (POP3) client. The user must be sure to configure the Reply-to address as email@example.com.