Problem

I have successfully set up a Postfix mailserver with an SMTP interface, set up DKIM, DMARC, SPF, all that stuff.

But I had a problem when testing the delivery of emails to Gmail.

Namely, SPF would pass if I sent it by the sendmail command from the server (some of the ARC-Authentication-Results header content of the delivered message in Gmail):

spf=pass (google.com: domain of [email protected] designates xxx.xxx.xxx.xxx as permitted sender) [email protected];

where xxx.xxx.xxx.xxx is an IPv4 address of the server

but would fail if I connected to the server using SMTP and sent it that way with:

spf=fail (google.com: domain of [email protected] does not designate yyyy:yyyy:yyyy:yyyy:yyyy:yyyy:yyyy:yyyy as permitted sender) [email protected];

where yyyy:yyyy:yyyy:yyyy:yyyy:yyyy:yyyy:yyyy is an IPv6 address of the server.

Fix

I realized that the SPF DNS record is only set up for the IPv4 address so I set it up that it also works with the IPv6 and it started passing as

spf=pass (google.com: domain of [email protected] designates xxx.xxx.xxx.xxx as permitted sender) [email protected];

where xxx.xxx.xxx.xxx is the IPv4 address of the server (weird)

Questions

1. Is there a way to force IPv4 when sending through the SMTP? Most machines nowadays should handle IPv6 but I would be happier if it didn't touch the IPv6 when I don't want to.
2. How is it possible that before it failed due to IPv6 not being present in the SPF DNS record but now it passes with IPv4 after I added IPv6?

If there are any settings needed to be known, let me know. There are many lines of config files so it wouldn't be practical to post them all.