So we were working on a project for a customer who has 6 Exchange servers, 3 CAS and 3 MBX. The customer have an Exchange Server 2013 organization and about 200 users.
Basically it all sounded easy and quick project, however their Exchange Server has other saying on this! After we configured the co-existence scenario on their Exchange Server, we had an issue where their local anti-spam application started to block incoming email messages from Google.
This is a normal behavior of course as those emails might be considered spoofed emails. So we added Google IP address ranges for outbound SMTP. Once we did that and some other settings on their Exchange Servers we were able to get their email traffic going between G Suite and on-premises Exchange Servers.
This was the easy part… However there was something else being cooked for us by the Exchange Server