Some Exchange Servers are unable to send mail to our IPv6 mail server

I manage about 300 mailboxes on a linux based mail server.

Our mail servers are IPv6 enabled and the configuration works correctly (we have no problem receiving messages from gmail accounts via IPv6)

A couple of times a year we get reports from people using self hosted exchange servers that they can't send emails to our customers - the messages are bounced.

Whenever this issue is reported it is always exchange, and the servers are always self hosted (hosted on a corporate LAN).

The messages are always bounced back for the same reason and our server's IPv6 address is always the endpoint that was attempted. This strikes me as odd because I find it very unlikely that these businesses have IPv6 internet connectivity.

Here is what the bounce generated by exchange looks like:

Remote Server at <OURCLIENTSHOSTNAME> (<OUR PRIMARY MAIL SERVER'S IPv6 IP ADDRESS>) returned '441 4.4.1 Error encountered while communicating with primary target IP address: "Failed to connect. Winsock error code: 10051, Win32 error code: 10051." Attempted failover to alternate host, but that did not succeed. Either there are no alternate hosts, or delivery failed to all alternate hosts. The last endpoint attempted was '<OUR SECONDARY MAIL SERVER'S IPv6 IP ADDRESS>'

In talking to the admins of these servers we consistently hit a brick wall and get told the issue must be at our end but it looks to me like the Exchange Server is resolving our AAAA record and trying to reach our server via IPv6 even though the Exchange server does not have a correctly configured IPv6 internet connection.

Can anyone shed any light on what might be happening here? If this is simply Exchange being poorly configured by the sysadmin are there some simple instructions we can give them to rectify the issue?

Thanks for reading.