Finally a public KB article about the infamous Event ID 7515 issue.
This article describes an event that may be logged when Intelligent Message Filtering (IMF) scans a message that is larger than 3 MB. IMF does not scan messages that are larger than 3 MB.
Note IMF is available as an add-in for the original release version of Microsoft Exchange Server 2003 and for Microsoft Exchange Server 2003 Service Pack 1 (SP1). However, IMF is included with Microsoft Exchange Server 2003 Service Pack 2 (SP2).
After you enable the IMF add-in in Exchange Server 2003 or the IMF feature in Exchange Server 2003 SP2, the following event may be logged in the Application log:
Event ID: 7515
Category: SMTP Protocol
Type: Error Message : An error occurred while Microsoft Exchange Intelligent Message Filter attempted to filter a message with ID <ID_Number>, Priority_Number From smtp:E-mail_ID and Subject Subject_Text. This message will not be filtered. The error code is 0x800710f0.
This event is logged when IMF tries to scan a message that is larger than 3 MB. In this scenario, you can safely ignore this event. When this event is logged, the message scan fails. The message passes through IMF without being scanned.
IMF is not designed to scan messages that are larger than 3 MB. Typically, spam e-mail messages are not larger than 3 MB because it costs more to send large messages.