Is running a mailserver with ipv6 address as the primary static ip possible?

Solution 1:

It is definitely possible - you don't have to change much, you just need to update the DNS entries for your mailserver to point to that IPv6 address.

But whether you really want to do it, is another question... there are still tons of mailservers out there which do not have the means to contact an IPv6 address - none of those domains would be able to send an email to you!

I would rather go with a couple of mailrelays: you have a number of public ipv4 mailservers which do nothing but forwarding emails to the correct server, which stands in a private subnet.

Or you go with an hybrid aproach - mailrelay for ipv4 and directly accessible via IPv6. But if you use IPv4, I would really recommend using a static IP.