I don’t think you need to disable the entire IPv6 stack to do this. It’s dangerous and can have negative effects (probably Sieve won’t work anymore for instance). However, if you just want to make that your outgoing mail uses IPv4, you can edit or create data/conf/postfix/extra.cf
and add the following lines:
smtp_address_preference = ipv4
smtp_bind_address = [your_ipv4_to_use_for_sending]
The restart the postfix container docker compose restart postfix-mailcow
. Your ourgoing mail will now no longer use IPv6 but IPv6 remains enabled for all the rest.