Exchange 2013 front end and Exchange 2013 EDGE in the environment.
The mail flow should go from front end exchange to edge and to interne but in some strange cases front end skips the edge server and recipient from Google recieves the directly from frontend with internal ip and it rejects the message because it can not find the SPF.
Error message output (and option to get ipv6 from ISP is unfortunatelly not an option :( ):
mx.google.com rejected your message to the following email addresses:
'Nuket Karagulle' (nuketkaragulle@gmail.com)
Your message wasn't delivered due to a permission or security issue. It may have been rejected by a moderator, the address may only accept email from certain senders, or another restriction
may be preventing delivery.
mx.google.com gave this error:
[2605:e000:5b0c:b600:217b:9d84:ff0f:e17a] Our system has detected that this message does not meet IPv6 sending guidelines regarding PTR records and authentication. Please reviewhttps://support.google.com/mail/?p=ipv6_authentication_error for more information. fd9si12461664pac.80 - gsmtp
bostjanc