RFC5322[0] in section 2.1.1 mandates that the line cannot be longer than 998 characters and should not be longer than 78 characters (excluding CRLF). When we use raw_data_manager (default for EmailPolicy, EmailMessage) it does the correct thing as long as the message contains characters outside of 7bit US-ASCII set - base64 or qp Content-Transfer-Encoding which respects the line wrapping at 78 characters.

However if our message is limited the characters from the 7bit US-ASCII set no transfer encoding is applied, and such messages can easily go beyond 78 or even 998 characters.


[0]: https://tools.ietf.org/html/rfc5322.html#section-2.1.1