i've tried all sorts of settings. here's a reply from our other hosting company (who we're using for our mail)
If you are required to use an SSL connection you would wan to use the settings I will include below. You may also want to check with them to make sure they are not having an issue with the SSL certificate used for the mail services. We use a self-signed certificate which on some services produces a warning I am not familiar with mattermost enough to say however this should be a fairly quick question for them to answer.
please can someone help this is proving to be a very troublesome setup