You must set the SSL settings to “TLS”:
gitlab_rails['smtp_enable_starttls_auto'] = true
gitlab_rails['smtp_tls'] = true
And if you are on a system with outdated CA certs also need:
gitlab_rails['smtp_openssl_verify_mode'] = 'none'
If you get authentication errors(Dont worry, it says plain but it is over TLS):
gitlab_rails['smtp_authentication'] = 'plain';