Setting auto expunge off
If you have already set server-side sorting, activated the squatter
indexing process and any other minor performance hints and still have serious delays browsing >1K-message mailboxes, try the following: on the configuration menu, set the option Folder Defaults > Auto Expunge to false.
Carlos Pérez (carlos dot perez at tiscali dot es) 2005-07-14
Courier IMAPd configuration
At least with the Debian Woody courier-imap package /etc/courier/imapd ships with:
# Maximum number of connections
# to accept from the same IP address
Since by default SquirrelMail connects to 127.0.0.1 this could be a problem if you have more than 4 simultaneous users.