Transact SMTP supports HTML, text, and multi-part messages. The message content type and encoding are important for Transact to determine the content of the originating message. Transact has to interpret HTML as HTML, text as text, and both for multipart.
This is specific to Transact. The key to this content type is encoding, as shown in the message header best practice examples below.
Transact message header best practice examples:
HTML message headers
TEXT message headers
Example of what does not work in an HTML message header:
In the above example of 'Base64 transfer encoding,' Transact does not recognize the body as being HTML, and the sent message is unreadable by the customer.