Using sub-domains for Mail routing can cause issues in the following circumstances:
If the sub-domain is called exchange.domain.com and the Foreign Domain is also called Exchange, mail can end up looping and never get delivered to Exchange.
This is caused by a limitation in Domino where a sub-domain and a Foreign Domain of the same name can cause issues.
The resolution is to rename the Foreign Domain to something else, making sure to change the matching entry in the Calendar Domain entry for the Exchange users.
