I have been through Microsoft regarding this issue already. They claim there is no problem there end because SMTP works in my Thunderbird, my network printer and in outlook. From that, they won't offer any more support.
I've tried to setup mattermost the same way Thunderbird is setup. With STARTTLS, and it's not working unfortunately. Can someone tell me which script, or function mattermost uses to send emails? I will look around for similar issues.