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Is it possible to configure Sendmail to forward the message to another server instead of returning it as NSU (no such user)?

We are in the process of migrating from a sendmail server to Exchange 2010. Due to various reasons it is impossible to cut over the entire domain at once. (600 mailboxes, 21 locations full of people who often have trouble even turning a PC on, and only 3 support staff to hold hands)

We are also changing our email address format at the same time, so this is the plan in my head:

-We create the mailboxes on Exchange using the new format of first.last addresses (roy.urick@) but DON’T create the current aliases yet, currently rurick@. (Creating the aliases of the current production email addresses would cause delivery issues between new exchange mailboxes and not yet migrated users as ALL of the mail from exchange users would stay local and not be delivered to the old mailboxes)

-We would configure our current sendmail server to somehow NOT return no such user (NSU) errors but instead forward the message to the Exchange server for further processing.

-On the old server we would .forward the old mailbox (rurick) to the new (roy.urick) address as we migrate each user by hand.

-When a message is received at the old server for roy.urick (or any other nonexistent mailbox) it would be forwarded to Exchange (and that server would determine whether it should be delivered or returned as NSU).

-If the message was for rurick@, the .forward would take over and the previous step would happen.

-Then eventually once all of the mailboxes were migrated, we would update the aliases on the new server to make the old addresses work, shut down the old server and change the MX record to the new server.

Any other suggestions?

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In your add:

define(`LUSER_RELAY', `[some.other.server]')dnl

Then rebuild your and restart sendmail. In Debian I do this by running sendmailconfig. Other Operating Systems and Distributions have different ways of doing this.

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