Comment 5 for bug 1518051

Both GMail and Outlook allow signing in to their IMAP and SMTP servers via Oauth2. Dropping Geary's IMAP server altogether seems like an overkill solution, given that work only needs to be done on the authorization components of Geary. The rest is fine.

Contact integration would require additional work though.

Outlook: https://msdn.microsoft.com/en-us/library/dn440163.aspx
Gmail: https://developers.google.com/gmail/oauth_overview