I'm runninig SmarterMail in the free version (thanks for that!) on a full blown dual stack DSL internet connection (Deutsche Telekom) behind a router, that supports IPv6 quite good (AVM FRITZ!Box). But after a reconnection (normally after some weeks) the public IPv4 address changes as well as the IPv6 prefix /56. This is supported by AVM's dnyDNS-Service, where I can map a hostname not only to the router's addresses, but even to the server's public IPv6 address, which is really routed - no NAT for IPv6 as with IPv4. Even my mobile mostly uses IPv6, since the AAAA record exists - what I can't change
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The private part of the server's address is fixed, but when the public prefix changes, the services (IMAP, SMTP) lose their bindings on IPv6, luckily the IIS dosen't, as I can bind it to all addresses. I could imagine some possible solutions:

- an option to bind the services to all addresses like IIS

- specify an IPv6 binding only by the private suffix, that dosn't change

- as a workaround a possibility to change/add a binding via CLI, which can be done by a script

Of cause switching off IPv6 would help also, but you can read everywhere: DON'T