imap proxy limited to about 210 connections

[bidwell]

I have nginx proxying imap and pop between 3 different backend servers, but
it seems to be limited to about 210 concurrent connections.  Requests beyond
this get a connection timed out.  I tried adding more worker processes but
that didn't do anything.  I have multi_accept on and have raised the number
of worker_connections, but still no luck.  I see rate limiting and
connection bandwidth limiting, but these appear to apply to the http
protocol and not the imap/pop protocol.  What parameters to I adjust to
increase the number of concurrent sessions to imap/pop?

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,246642,246642#msg-246642