> We inform you that, at OVHcloud, it is not possible to purchase and reserve a block of IPv6 because each IPv6 is unique is associated with only one host.

Thank you for contacting Spamhaus CSS Removals,

This is to be expected of VPS hosting.
Consider abandoning IPv6 for mail. The merits are insignificant.

Even with my own /64, getting places to accept mail requires working DNS and that is beyond OCN...

Regards,
R e

main.cf
##
# The setting "smtp_address_preference = ipv4" is not a solution for remote servers that
# flag email received over IPv6 as more 'spammy' (the client IPv6 address has a bad or missing
# PTR or AAAA record, bad network neighbors, etc.).
# Instead, configure Postfix to receive mail over both IPv4 and IPv6, and to deliver mail over only IPv4. 
#
# Example:
#
#   /etc/postfix/main.cf:
#       inet_protocols = all
#
#   /etc/postfix/master.cf
#       smtp ...other fields... smtp -o inet_protocols=ipv4
#
# The setting "smtp_address_preference = ipv6" is unsafe. All deliveries will suffer delays during an IPv6 outage, even while the destination is still reachable over IPv4.
# The setting "smtp_address_preference = any" is safe.
# 
#
# https://serverfault.com/a/1049862/225489
# https://www.postfix.org/postconf.5.html#smtp_address_preference
# 
inet_protocols = all
smtp_address_preference = any

smtp       unix  -       -       y       -       -       smtp
    -o inet_protocols=ipv4