This looks like a useful feature and seems simple to add - we'll consider it for one of the next releases. I think we will extend the
Ews API instead of the
MailMessage class (because deferred sending is an EWS feature rather than MIME feature). If that doesn't suit your needs, let us know.
As for the
StoreMessage behavior, we'll look into this. I guess that the interaction of 'from', sender', 'on behalf of' with shared mailboxes is somewhat tricky.