Are you sure the terms of service for office 365 allow this? Discourse can send potentially thousands of emails per hour on a busy instance.
For example, using Gmail for your outgoing Discourse mail service is absolutely prohibited by the Gmail terms of service. I would be quite shocked if this was not also the case for Outlook / Hotmail / Office365.
(For incoming mails, e.g. reply-by-mail it’s less of a big deal since the volumes are so low.)