2020-10-15 12:51:45 submission(mua@example.com)<8OJP+rqxuvho7Z95>: Info: Successfully relayed message: from=<test@remote.example.com>, size=84, id=LMe...Aw, nrcpt=1, reply=`247 2.0.0 Ok: queued as 4CC0KY1wXNzWf93'

not fatal, but wasted effort, and noise in the logs.

how/where do I configure (just) the dovecot smtp-client -> submission_relay_host to only connect IPv4?

It appears your host has A and AAAA records in your DNS. The clients will try IPV6 first if they see an AAAA record.

If you don't need IPV6 for your host remove the AAAA record. All connections will then only use IPV4.

If you need IPV6 for some other reason then create an alias DNS A record and point your clients to that instead

e.g.

myhost  A    192.0.2.1

	AAAA 2001:db8::1

myhostv4 A   192.0.2.1