When you originally setup your SmarterMail server with your IPV4 e-mail addresses, you setup an rDNS for the fully qualified domain name of the server.
From the error message, it appears that you have not properly setup an rDNS for your new IPV6 IP address.
Once you do that, and update your FEEDBACK LOOPS with YAHOO and the other providers, these errors should stop.
IPV6 is so new that this is a commonly overlooked error.
If you have SmarterMail upgrade protection, you will probably want to consider upgrading your software to 11.4.5784, which was released last night and contains some very significant security updates.
It also handles DMARC, DKIM, SPF and other things which are now very important to ensure e-mail is accepted by everyone.
Phone: (224) 444-0169
E-Mail and DNS Security Specialist
Network Security Specialist
Customer Service Portal: https://portal.chicagonettech.com
Security Blog: http://networkbastion.blogspot.com/
Web and E-Mail Hosting, E-Mail Security and Consulting