No TestFlight invite email gets sent for internal users

Question

I set up my application for TestFlight. For external users it is waiting for review. But if I want to invite internal users, I select them and press invite. iTunes Connects says

Answer 1

I have had the same issue. 3 accounts altogether (1 mine, 2 others) initially, mine worked and both of the others. When I tried to update, only one worked. Since the only similarities with the other 2 was that they were GMAIL accounts, so I added one of my apps (to my phone, and I had to log onto test flight using it) - all seems to be working. I was told the Google spam filter was filtering these out and I couldn't figure out how to allow them through.