After backing up and restoring a site collection, incoming e-mail for the site collection no longer works.
In the ULS log, the following message appears (where mailalias is the name of the e-mail alias):
The Incoming E-Mail service has completed a batch. The elapsed time was 00:00:00. The service processed 1 message(s) in total. Errors occurred processing 1 message(s): Message ID: <0b7d0469-8bdc-4d9c-9834-aada5c458037@server> The following aliases were unknown: mailalias
In this case incoming e-mail for the alias is associated with an event handler/receiver.