I'm still missing the capability to send out emails over SMTP to IPv6 enabled MXs.
Only a little change is needed, to make this feature available:
https://community.sophos.com/products/unified-threat-management/astaroorg/f/56/t/49376
Here on our UTM this is "enabled" for about one year now, without any problems. (native IPv6 connection; Deutsche Telekom).
For production use there should be a new Option under Email Protection -> SMTP:
[x] Prefer IPv6 when sending emails
Of course, this option is needed, as not everyone with IPv6 connection can set the required PTR-Records.
I'm looking forward, to see this feature in 9.2 [:)]