Mattermost wont connect to postfix

Hi all!

I followed https://docs.mattermost.com/install/smtp-email-setup.html to set up mail notifications in my mattermost, which is run in a docker.

I also have postfix running, and can successfully send emails through it.

However, when I try to "test connection" in the email settings, I get this error in the logs

[2017/03/12 04:41:37 UTC] [EROR] /api/v3/admin/test_email:SendMail code=500 rid=cu7knu9z5tndpx8uii3mkbe3dc uid=77mxq5aib7y9jjjqkr4ppcrdea ip=172.18.0.1 Failed to open connection [details: dial tcp [::1]:25: getsockopt: connection refused]

I'm very stuck!

Thanks