THE OFFICIAL BLOG OF THE SBS "DIVA"

Be very careful when you do this. If you are receiving mail directly on your Exchange server and the public DNS world thinks your Exchange server is authoritative for your e-mail domain, this setting will create a HORRIFIC mail loop. Allow me to explain using Susan's example:

1) Someone inside your organization sends mail to an address that doesn't exist in SBS/Active Directory/Exchange.
2) Your SBS server looks to see if it knows about that mailbox. When it doesn't, it sends the mail to the host you specified (in this case, your ISP's SMTP server).
3) Your ISP looks at the mail and determines that it isn't authoritative for the domain and that you are allowed to relay through it (because you are on their network), so it looks up in DNS for the server responsible for that e-mail domain (in this case, your Exchange server).
4) Your ISP's SMTP server sends the mail back to your Exchange server.
5) Go back to step 2 and repeat ad infinitum - very rapidly and probably bringing both your ISP's mail server and your Exchange server to their knees.

The only way this will work is if the server you are forwarding to thinks it is authoritative for the mailbox in question and/or if public DNS points your e-mail domain somewhere other than your Exchange server.

Be afraid if you set this - be very very afraid if you don't know what you are doing.

Here is the article from the MS knowledgebase that discusses the proper way to handle this situation:

http://support.microsoft.com/default.aspx?scid=kb;en-us;321721

I just want to emphasize two things:

1) You only need to do this when using the POP3 connector. If you are using SMTP mail... DO NOT use it.

2) KB311721 is basically a different (and more complex) way of doing the same thing (although it gives you more options). But again, there is no "proper way" to do this if you are using SMTP... just don't do it.

I'm not sure I agree with Javier that this shouldn't be done with SMTP - the article I posted specifically outlines how to handle the situation. The forwarding option is a much more dangerous method of configuration, given that many people won't read the caveats about only using it if you are using the POP3 connector.

I have had at least one client that needed the ability to host mail on their Exchange server for one group of people in a domain, and on an external server for another group of people. I have had other issues with the MS POP3 connector, so I wasn't willing to have the external host be completely authoritative for everything.

So, we configured the setup using the KB article. We pointed external DNS at the Exchange server and had it route any mail for that domain that it didn't know what to do with to the mail server that hosted the other POP mailboxes. It worked perfectly.

If someone follows these instructions when they are using SMTP, the consequences will be disasterous. In this case I would argue that it makes more sense to use the MS supported methodology in the KB article.