If you want to use ipv6 make sure you don’t share the /64 subnet with somebody else, in my case (OVH vps) they gave me a single ipv6 address, and my mails weren’t going through because another address in the range was blacklisted, see this report by Spamhaus:
According to current agreed upon standards, any given /64 is intended to be one single entity.
The industry standard for the smallest IPv6 allocation to individual customers, even for home-uses like cable, DSL or wireless is a /64.
Some providers choose to not follow the industry standard, and allocate smaller subnets. As a result, you are sharing the same network segment as other customers, which can include problematic ones.
This non-standard allocation practice can result in IPv6 IPs being listed in CSS because of neighboring IPs that share the same /64 - contrary to industry standard.
NOTE: If your allocation is smaller than /64, we cannot remove it from CSS, and the situation needs to be corrected with the provider prior to requesting removal. For more information about IPv6, IP allocation, and industry standards, please see our FAQ.