Email notifications to Exchange Server

We are trying to evaluate Mattermost for company messaging. I’ve setup and installed on Debian 8; everything is working fine except for email notifications. When I test the following I receive 2 errors:

TLS on 25: Failed to open TLS connection [details: tls: first record does not look like a TLS handshake]

STARTTLS on 587: Failed to authenticate on SMTP server [details: 504 5.7.4 Unrecognized authentication type]

The only way I’ve received successful test notifications via email is by using no encryption (PLAIN)

I did see how this was mentioned with regards to Office360 but we use an in-house Exchange 2013 server. Has anyone has luck getting this to work?.

Exactly the same issue over here. We get successful notifications only if set to port 25 with no security. This is definitely not how we want it configured. We’re using Exchange 2013 and have tried every possible combination of settings to no avail. It’s surprising that there isn’t more chatter about this anywhere.

It looks like there is relevant info inside