We are experiencing inconsistent IMAP functionality. There are some posts in this forum, but I can not find any related symptoms in my case. Mainly problems are connected with iPhone users (ZCS OpenSource) via regular IMAPs connection. According to firewall logs, user gets connection with NGINX proxy (Separate server), but after this - no logs at all. Mailbox - silent, NGINX - silent. iPhone client shows "connecting". After a period timeout disconnect phone from server.
At the begining, we were using statefull packet inspection switched on for IMAPs connections. Things improved, when we switched off SPI, but still there are the same symptoms. The difference is, when connection is successfull, it loads faster, than before.
All DNSes resolve corectly, all servers are reachable, even, if opening all ports from all ZCS servers. Switching phone off/on first sync is OK. After, there is a stale state again. Each of our servers ir not less than 2GB RAM and 4 CPUs.