It would be nice to be able to send messages with an importance.

MailKit exposes two interfaces for this: https://github.com/jstedfast/MimeKit/blob/master/MimeKit/XMessagePriority.cs

and https://github.com/jstedfast/MimeKit/blob/master/MimeKit/MessagePriority.cs

I'd like to be able to use one API so it sets both of these headers (for legacy support).