After an upgrade of SPF in Gmail servers, seems that Gmail is blocking e-mails with dynamic IPv6 addresses which have not PTR records in the RDNS. Because of that Gmail is also checking for every IPv6 address that is containing in the e-mail headers and not only the IP that is assigned on this server (the IP of the server that sending the e-mail to the Gmail’s mail exchanges (MX)).
The solution is to completely disable the IPv6 addresses on the Edge servers (MX) of your Exchange Infrastracture. See below:
1. Regedit and go to HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\TCPIP6\Parameters\
a) Edit or Create a DWORD (32-bit) with the name “DisabledComponents”
b) By default the value is 0x00000000 (0), you have to Right click and modify this value with the Hexademical 0xffffffff (you have to erase the 0 and write only the “ffffffff“), in Decimal it is the “4294967295” value.
2. You have to Restart your servers (edge exchange servers).
3. If you want to verify that you have set it up correctly just open a cmd and run this:
a) C:\> reg query HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\TCPIP6\Parameters /v DisabledComponents
b) The output you must take is this: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\TCPIP6\Parameters DisabledComponents REG_DWORD 0xffffffff
That’s all. Have a nice day.