A. You have several options for connecting an Exchange Server mail system to another mail system. You can use these options for connecting Exchange to most mail systems (e.g., Netscape Mail).
You can configure the Exchange server as authoritative for a particular SMTP domain (e.g., savilltech.com). Then, when the Exchange server receives an email message for a user who isn't in Active Directory (AD), Exchange can forward the message to an alternative mail system, such as a Netscape mail server. This method works as long as the users in the other mail system don't exist in the AD directory. If the non-Exchange users are in AD, Exchange finds them but can't find any mail information for them. Consequently, Exchange generates and sends a nondelivery report (NDR) to the message's originator.
If all recipients are in AD but the mailboxes are divided between the two mail systems, create a second SMTP email domain--for example, netscape.savilltech.com--on the non-Exchange mail system. For the AD users whose mailboxes are on the other mail server, mail-enable the users (i.e, create email address attributes for them in AD), make their primary address email@example.com, and configure firstname.lastname@example.org as a secondary address. (Don't confuse mail-enabling a user with mailbox-enabling the user. You don't want to create a mailbox for the non-Exchange users; you just want to create their mail attributes in AD.) Then, create an SMTP connector with a namespace of netscape.savilltech.com to forward mail to a smart host--in this case, the Netscape mail server.
On the non-Exchange mail system, you can add accounts for users who have mailboxes on the Exchange server and specify the Exchange server as the mail host for those accounts. If necessary, you can create a second SMTP email domain (e.g., exchange.savilltech.com) on the Exchange server. However, you don't have to create another SMTP email domain when the alternative mail system is Netscape, as in our example, because Netscape lets you configure a mail host for each mail object.
When you receive an email message from a Netscape user, the sender's address is email@example.com. If you reply, Exchange will try to send the message to firstname.lastname@example.org. Therefore, you must add a secondary address with @savilltech.com on the Exchange side for the Netscape users so that the reply attempt doesn't fail because the Netscape user doesn't have a mail account on Exchange. When you have a secondary address of email@example.com, Exchange matches the mail address to the account in AD, and if you reply to the message, Exchange sends the reply to the user's primary address (firstname.lastname@example.org), which solves the problem.