If you are wondering how senders are managing the transition to IPv6 delivery, in particular as it relates to rate limiting, you are not alone. As an example, Comcast rate limits v6 differently than v4, and doesn’t appear that senders can make a real distinction across hundreds of VirtualMTAs. As a sender if you utilize the global domain definition for one ISP, (i.e. comcast) that definition applies to all VirtualMTAs. So, if you add IPv4 and IPv6 to the same VirtualMTA, IPv6 usage will take precedence over IPv4 in a default OS/Network setup.
The default policy table gives IPv6 addresses higher precedence than IPv4 addresses. This means that applications will use IPv6 in preference to IPv4 when the two are equally suitable. An administrator can change the policy table to prefer IPv4 addresses if he/she chooses. This is part of RFC 3484, the algorithms specify default behavior for all IPv6 implementations. The behaviors however can be modified and do not override choices made by an administrator on applications or higher level protocols as stated in the RFC.