Asking user to type in “_submission._tcp” seems like a poor workaround. This being Autodiscovery it’d be logical for Postfix to automagically derive SRV lookup address based on relayhost setting.
“relayhost = example.com” is an obvious candidate for SRV autodiscovery if an MX lookup fails or if SRV is prefered over MX. With an optional switch to disable “relayhost_autodiscovery”.
