That seems to have done it, thank you very much.
I’ve been stress testing a dev system for a week now. Without no option http-use-htx I would start getting mapi connection errors (client side, the outlook can’t connect to mailbox error) after simulating 600 clients connecting for 8 hours. After adding no option http-use-htx haven’t had problem since.
The funny thing is that testing it small scale, with htx, like from a couple of workstations , everything would work fine, when hitting the servers with more clients, as in my stress test, it would suddenly break and stop connecting to the mailbox.