Sometimes I get this message in the log file:
2013/05/14 13:13:42 [info] 2850#0: *50314 upstream sent invalid response: "* BYE zimbra.foo.bar Zimbra IMAP4rev1 server closing connection; service busy" while reading response from upstream, client: y.y.y.y using starttls, server: 0.0.0.0:143, login: "email@example.com", upstream: x.x.x.x:7143 (y.y.y.y:42820-x.x.x.x:143) <=> (x.x.x.x:60527-x.x.x.x:7143)
If that happens, the client cannot connect to the server. It usually means, that it cannot send a mail or save it (automatically) to Drafts folder. After a couple of retrying usually in the end it succeeds.
Earlier there was a similar problem and increasing pool limits helped. But this time there are no similar error messages, only this.
Do you have idea?