i'm getting troubles with mail clients & zimbra imap: very often outlook shows error "imap4rev1 server closed connection; service busy", after which it cant connect to server until restarted several times; almost same thing with thunderbird - shows that server imap server closed connection and starts working only after hit "ok" many times.

I've set imap-related events loglevel to debug, but got no errors in mailbox.log;
the only errors i've found in any logfiles were in nginx.log (but they just copy ones that shows in clients":
"*5199 upstream sent invalid response: "* BYE post.mycompany.ru Zimbra IMAP4rev1 server closing connection; service busy" while reading response from upstream, client:, server:, login: "somelogin", upstream:, [] <=> []"
server's load average not exceeding 4.0

Please help in solving this problem.