The below instructions are how to ensure that mail flow works when migrating mailboxes from one forest to another. In this example, emails are going to the contoso.com domain which has mailboxes in the asia.contoso.com domain in the contoso.com forest and the litwareinc.com domain which is the root domain of a separate forest.
After a mailbox is moved from the source forest, the mailbox is replaced with a mail enabled user which then redirects the mail to the mailbox in the target forest.
Source Exchange server: contasex01.asia.contoso.com
Target Exchange server: litex01.litwareinc.com
To ensure that mail can be delivered to all mailboxes, no matter which forest they are part of:
- Set up an internal send connector on contasex01 to send email for contoso.com to litex01
- Set up an internal send connector on litex01 to send email for contoso.com to contasex01
- Ensure the receive connectors on both contasex01 and litex01 are configured for anonymous authentication and will accept connections from the other server.
- Set up a new accepted domain on litex01 for contoso.com and ensure it is not authoritative but set to internal relay. Internal relay means that when litex01 receives an email for a contoso.com address, it'll try to deliver to the contoso.com recipient in its forest and if it doesn't find a valid recipient, it'll send it through the contoso.com send connector to contasex01.