Ok, they include it in the Received line. Something like this in user1's .procmailrc should deliver matching mail automatically to user3. The reason I don't use the To header is it that it won't catch mail that's been cc'd or bcc'd to user3@yourdomain:
Code:
:0
* ^Receved:.*lotus.websitewelcome.com.*for.*user3@yourdomain
! user3@localhost
