Update: I got the activation email working using the port 587 and using the following value in app.yaml file:
DISCOURSE_SMTP_ENABLE_START_TLS: false
Once I logged into the admin interface I could see the following Sidekiq errors (from before?):
Jobs::HandledExceptionWrapper: Wrapped OpenSSL::SSL::SSLError: hostname “mail.mydomain.com” does not match the server certificate
Now, I understand that this error went away because I set the DISCOURSE_SMTP_ENABLE_START_TLS to false, but is this the right way to do things?