I would like to be able to specify the smtp protocol from the user interface.
Specifically, the solution to the problem described here on the official postfix documentation:

https://www.postfix.org/postconf.5.html#smtp_address_preference

The setting "smtpaddresspreference = ipv4" is not a solution for remote servers that flag email received over IPv6 as more 'spammy' (the client IPv6 address has a bad or missing PTR or AAAA record, bad network neighbors, etc.). Instead, configure Postfix to receive mail over both IPv4 and IPv6, and to deliver mail over only IPv4.

/etc/postfix/main.cf:
inetprotocols= all
/etc/postfix/master.cf
smtp ...other fields... smtp -o inetprotocols=ipv4