Hanspeter Kunz put forth on 4/24/2010 9:17 AM:
yes, that is what I suspected too first. Indeed, I had to increase the max_userip_connections limit in the past. Since then, our dovecot server works fine with all clients (Evolution, TBird, Applemail, ...), except for roundcube. And there is no indication in the logs that the max_userip_connections limit is hit.
This may not be an imap concurrency limit being hit but an auth process limit. The two are configured separately and auth has many more knobs.
Regardless, installing an imap proxy will very likely solve your problem painlessly and with little effort.
Please do install and properly configure an imap proxy on your RC server. It's very lightweight. You don't need a separate box for it. This Debian aptitude description pretty much spells out why (most) webmail servers need an imap proxy.
Description: IMAP protocol proxy UP-ImapProxy proxies IMAP transactions between an IMAP client and an IMAP server. The general idea is that the client should never know that it is not talking to the real IMAP server, but ImapProxy caches server connections.
ImapProxy was written to compensate for webmail clients that are unable to maintain persistent connections to an IMAP server. Most webmail clients need to log in to an IMAP server for nearly every single transaction; This behaviour can cause tragic performance problems on the IMAP server. ImapProxy tries to deal with this problem by leaving server connections open for a short time after a webmail client logs out. When the webmail client connects again, ImapProxy will determine if there is a cached connection available and reuse it if possible. Homepage: http://www.imapproxy.org