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Instructions to configure Exim (on Linux) as a backup MX for our Exchange 2003 Primary MX?

Posted on 2006-11-15
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Last Modified: 2010-08-05
We have an Exchange server that is MX 10 for our mail domain exampledom.com. We have another server which is running Linux and has Exim up and running, hosting mail for a number of domains. The Linux server also has the primary DNS for exampledom.com. We would like to configure the Linux box to be a secondary MX for the domain. My understanding is that there is a way to configure Exim so that it will spool mail sent to exampledom.com in the event the Exchange server is unavailable, but will not store it in mailboxes. When the Exchange server comes back up, we'd like the spooled mail to be offloaded onto the Exchange server so it can be stored in the recipients' mailboxes. We've set the Linux server as MX 20 in the exampledom.com DNS, but when we test the failover (by stopping the exchange SMTP, or unplugging the ethernet cable) all mail to the domain is bounced by MX 20 with a "no such user here" error. The Exim server is part of a virtual private server package we have with a hosting provider(we have root and are free to make configuration changes). I'm not sure if the problem is that Exim somehow believes it is the "owner" of exampledom.com and therefore wants to store mail for it rather than to continually retry sending it to MX 10, or what. We can add users on the Linux box to match the destination users on the Exchange, but then their mail will be delivered to POP mailboxes in a failover situation, and that won't work for us in this situation. Any advice would be greatly appreciated. Thanks in advance.
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Question by:networxplus
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nitadmin earned 250 total points
ID: 17962334
What you describe is normal behavior.

You setup two MX records for your Domain.

mx1.domain.com 10    exchange server
mx2.domain.com 20    linux server running Exim.

If the mx1.domain.com is not reachable then e-mails will be sent to mx2.domain.com. This server better receive the e-mails. Otherwise then e-mails will bounce back with NDRs.

This is what you should do. Setup and configure mailboxes on your Linux server.
Then add a pop3 connector to your exchange 2003 server to retrieve the e-mails which end up on the linux server, whenever the exchange sever is unreachable.

If you have Windows 2003 Small Business Server, you can setup pop3 connector. If you have Exchange 2003 installed on a Windows 2003 server standard or enterpise edition, then you will need to get a thrid party pop3 connector.

Cheers,
NITADMIN
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by:MATTHEW_L
MATTHEW_L earned 250 total points
ID: 18065617
I have never really liked Exchange POP3 connectors they have always been troublesome.  If there were another way such as ETRN or just using the exchange server as a smart host for the linux host that would be a better solution that using a pop3 connector.
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