Messages with an Unknown Main Class are ones which do not fall into one of the other Main Classes.
Messages with a Main Class of Unknown can have any of the following Sub Classes:
The filter was disabled when the message was received, so no filtering was done.
The sending server attempted to send the message over multiple connections, which is not supported.
The sending server disconnected before finishing the message delivery.
The message could not be delivered to all recipients in the same connection, because of a conflict with the recipients' settings. The message will be retried for some recipients.
The destination mailbox did not exist.
The sender's address did not exist, and the settings required a valid sender mailbox.
The sending server has exceeded the maximum amount of concurrent SMTP connections that can be made in the time limit.
The Extra Class for messages with this Sub Class will show the number of concurrent SMTP connections and the time limit.
E.G. 10 / 1m / recipient