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Have the same issue after upgrading to 2018.3.0 on all ports 25, 465 & 587. Currently have a support case open.
The only option which worked was using the google restricted SMTP server "aspmx.l.google.com" on Port 25, as this does not rely on TLS. This is not a workable solution though as it will only send emails to Gmail or G-Suite users.
Having this same issue with GMAIL, as well as with Office365. When using port 465 error is cannot reach smtp port = -1; when using port 587 error is must issue starttls command. Cannot locate option for starttls