Probably not/maybe.

if we are talking mx based email delivery, then it would try ipv4 if it 
could not contact the server via ipv6 (so if you use a firewall rule).
But surgemail rule to bounce the address won't work as it will take that 
as  a hard failure and not retry.

if we are talking smtp sending from a customer (an authenticated user), 
then no that won't failover to ipv4 that would be sending to a specific 
address.

     ChrisP.

On 28/01/2016 5:59 a.m., surgemailHIDDEN@etwinsite.com wrote:
> If this blocks the ipv6 connection.. will the sender server try to send IPV4?
>
> What are the rules.. if ipv 6 is not available?