This would certainly include encryption, and it might include other means of securing messages. Appended files are banned. All documents are sent as part of a pair with their corresponding headers. No longer will the receiver be required to strip the header from a file before converting. E-mail software which appends documents to headers is banned.
All parties support the same compression standard.
In In the meantime, do me a favor. Do not use an incompatible e-mail program. Keep track of what works and what does not work. If you compress a file, clearly identify the compression software you used. Send documents my software can read.