One of the new features added to the Microsoft Graph recently was the ability to create and send Mime Messages (you have been able to get Message as Mime for a while). This is useful in a number of different scenarios especially when trying to create a Message with inline Images which has historically been hard to do with both the Graph and EWS (if you don't use MIME). It also opens up using SMIME for encryption and a more easy migration path for sending using SMTP in some apps. MimeKit is a great open source library for parsing and creating MIME messages so it offers a really easy solution for tackling this issue. The current documentation on Send message via MIME lacks any real sample so I've put together a quick console app that use MSAL, MIME kit and the Graph SDK to send a Message via MIME. As the current Graph SDK also doesn't support sending via MIME either there is a workaround for this in the future my guess is this will be supported.
Pushing the Envelope in Messaging and Office 365 Development