FYI, if you’re having this error message trying to setup Mattermost SMTP with Mandrill on Digital Ocean try setting your port to 465 instead of the default 587.
Thanks argenisleon for the tip!
Hint for using Mailgun:
The only combination of authentication methods and port which worked for me is:
- port 2525 (or 25, but beware when using Google Cloud, because port 25 is blocked)
- STARTTLS
Whew! Thanks for this info. I was beginning to pull my hair out!