Invite staff doesnt send to gmail addreses

Freshly started ghost instance within digital ocean, mailgun set up.
Stuck with the problem, cannot send the invites to gmail addreses. Other domains works. Also, if adding members wirh gmail addreses, sending also works, Just problem communicationg with users - editors, contributors and writers. The same with revoking passwords, email sending fails.
Any idea where to look for the solution?