I have done some midification: On my email server, I have upgrading my Nginx from 1.6 to 1.9. I have add: http2 Both Nginx (email server and reverse proxy) have now the same Nginx version. But still have the same problem. Thx for your support. Posted at Nginx Forum: https://forum.nginx.org/read.php?2,271847,271858#msg-271858