Categorization on each message happens after a newly arrived message is put in the Submission queue. This type of content conversion is handled by the categorizer in the Transport service on a Mailbox server. Messages sent to recipients inside the Exchange organization don't require this type of content conversion. Message conversion for external recipients: This type of content conversion includes the Transport Neutral Encapsulation Format (TNEF) conversion options and message encoding options for external recipients. The types of content conversion that occur in Exchange 2016 and Exchange 2019 are unchanged from Exchange 2013: The decision to perform content conversion on a message depends on the destination and format of the message. Content conversion is the process of correctly formatting a message for each recipient.