I have done a number of web searches regarding subaddress use with Exchange.  Everything I have found says that Exchange does not support it, Office365 does not support it.  Now, since I can send emails with to addresses of the form user+modifier@domain to my Office365 address and since those emails do in fact show up in my user@domain Mac Outlook client inbox with the "+" to address intact, I'm inclined to think that either all of the postings were wrong or something has changed.

Be all of that as it may, does anyone know of a way to get the sender address on a given piece of email to include a subaddress modifier?

By the way, I have tested (though not exhaustively) subaddressing in both directions and am able to send to a modified address as well as receive modified address mail to me.