Which is the culprit; pfSense, Ubuntu, or MiaB?

If I have router announcements set to ‘assisted’ on the pfSense, the Ubuntu 18.04 device gets the IP address reserved for it in the IPv6 Server settings as well as it gets the SLAAC privacy address every 24 hours. If the Ubuntu has the SLAAC address, then MiaB tries to send to gmail with the IPv6 address. Of course, since I have the reserved address set in the DNS, gmail says I am violating their IPv6 rules (given the mismatch between the privacy address and the DNS address).

If I have router announcements set to ‘managed’ on the pfSense, the Ubuntu only gets the reserved address and does not ever get a SLAAC privacy address. If the Ubuntu does not have a SLAAC privacy address then MiaB only connects to gmail in IPv4. No mismatch between IPv4 addresses so mail goes through. But no IPv6 connections.

Is the problem with MiaB; it will only send IPv6 email if has a SLAAC privacy address? Or is the problem with the Ubuntu OS; if it doesn’t have a SLAAC address it won’t let MiaB use the IPv6 protocol? Or is it the pfSense router not handing out IPv6 addresses correctly; thus no IPv6 protocol use is possible.

Or is the problem actually between the chair and the keyboard?

While I also have Router Announcements on the pfSense set to ‘assisted’ which reportedly turns off SLAAC addressing, I decided to also tell the Ubuntu machine not to use privacy addresses.

Edited. /etc/sysctl.d/10-ipv6-privacy.conf
net.ipv6.conf.all.use_tempaddr = 0
net.ipv6.conf.default.use_tempaddr = 0

Rebooted. MiaB still seems to prefer to use IPv4 when connecting to gmail.

Anybody doing any testing of IPv6 only with their MiaB? If so, can we connect so that you can send email to me and I can respond so that you get what you need and I can confirm with certainty that other servers can IPv6 to me?