IMAP Proxy simply sits between your webmail server and your IMAP server. It accepts connections from your webmail server for each client login, then proxies that connection to your real IMAP server. When your webmail client disconnects, IMAP Proxy will leave the connection open to the IMAP server such that when your webmail client reconnects, the existing connection may be re-used.
IMAP Proxy is an extremely lightweight daemon written in C using POSIX threads. It compiles and runs on most current flavors of Unix, and it's available in pre-compiled package format for many distributions including Debian, Ubuntu, Gentoo and Fedora.
IMAP Proxy was originally written by Dave McMurtrie and released in 2002. As of 2010, it has been adopted by the SquirrelMail team.