I recently installed phpBB on my sever for a personal project and I was trying to get it to send emails. I had an issue using office 365 and while researching found that no one had figured out how to make it work with phpBB. I however am very familiar with Office 365 and think I have found the solution.
The server name needs to "tls:
(mxrecord)" of the domain, however as MX records are long there is not sufficient space in the admin console to add the full server name.
Before any fix is implemented in phpBB3, I can test this if you can give me the location where this data is stored in either MySQL or php files, or the page that the import field is on so I can make it accept more characters.