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.