IPv6 with Local Exchange sending Bounce Backs

We have a client who is on Comcast with a static IPv4 address and local Exchange.  Comcast has started rolling out IPv6 on everything and they are now receiving bounce backs when sending out (message below).  I've contacted Comcast and they say that they cannot assign a static IPv6 address, therefore we cannot set reverse DNS for IPv6.  I have added their IPv6 address to their SPF record which solved the issue temporarily, but since the IPv6 is dynamic it has changed, then they get bounce backs again.

We have a number of clients behind a hardware firewall, which we disable IPv6 on, and they are not having any problems as they are only communicating with the world on IPv4.

Any idea on a permanent fix to resolve this IPv6 dynamic address issue that inevitably causes bounce backs?

Your message wasn't delivered because of security policies. Microsoft Exchange will not try to redeliver this message for you. Please provide the following diagnostic text to your system administrator.
The following organization rejected your message: mx.google.com.
________________________________________
Sent by Microsoft Exchange Server 2007

Diagnostic information for administrators:
Generating server: SERVER.[xxxx].local
[xxxx]@gmail.com
mx.google.com #550-5.7.1 [2601:[xxxx]:c05c] Our system has detected that 550-5.7.1 this message does not meet IPv6 sending guidelines regarding PTR 550-5.7.1 records and authentication. Please review 550-5.7.1 https://support.google.com/mail/?p=ipv6_authentication_error for more 550 5.7.1 information. x75si3472751ioi.11 - gsmtp ##