We receive the same 2 errors from the Email Router every few seconds (I would use the numbered list here but it doesn't work correctly - like most of the features on MS forums):
1. For the same 2 mailboxes this error is thrown, yet there is nothing unique about them: An error occurred while checking for e-mail messages to process in mailbox
email@example.com. System.NullReferenceException: Object reference not set to an instance of an object.
2. An error occurred while opening mailbox firstname.lastname@example.org. System.Web.Services.Protocols.SoapException: The primary SMTP address must be specified when referencing a mailbox.