Here is my understanding of it -

Currently, TB uses POP3/IMAP protocol to access your Mailbox on Exchange Server.

Ideally, it should be done via MS EWS Web API [1][2] which TB currently doesn’t support, but MS Office Outlook does.

Given that MS Exchange is heavily used in corporate/company setup, it[3] will put TB on par with Outlook in that regard.

[1] https://www.techtarget.com/searchwindowsserver/definition/Exchange-Web-Services-EWS

[2] https://www.envisionup.com/blog/pop3-imap-microsoft-exchange-email-platform-use/

[3] https://bugzilla.mozilla.org/show_bug.cgi?id=1847846