Certain malformed MIME messages created with the Thunderbird mail client are treated differently when Symantec Mail Security for Microsoft Exchange (SMSMSE) 6.0 is running on an Exchange 2007 server.
When SMSMSE is not in place email messages with Malformed MIME cause Microsoft Exchange to send an NDR (e.g. "Diagnostic information for administrators.... #554 5.6.0 STOREDRV.Deliver; Corrupt message content ##"). When SMSMSE is installed no NDR is generated. The message is dropped.
The following events appear in the Windows Application Event log for each malformed message:
Malformed MIME encoding. This particular cause of malformed MIME encoding appears to be related to Mozilla Thunderbird and encoding of .doc and .pdf files, among others. Thunderbird misrepresents the format of the files as a macintosh file, and as a result our scanning engine crashes when attempting to scan according to the supplied format. The problem is that attachment files can be assigned the wrong MIME type by the Thunderbird mail client. Messages with the incorrect MIME type of "application/applefile" will be undelivreable by Exchange 2007. When a malformed message is received, the transport agent crashes, and an application event log entry is written. No message is received by the original recipient, and no NDR is sent to the original sender.
Upgrade to SMSMSE 6.5.
For customers who cannot upgrade to SMSMSE 6.5 at this time: the original source of the error is messages are being sent in a malformed condition by the Thunderbired email client. Customers are recommended to not use the Thunderbird mail client for sending outgoing messages if SMSMSE 6.0 is installed on the Exchange 2007 server.