This page explains how to set up SquirrelMail on Linux to access a remote IMAP sever. Web.de is used as an example

1. Download and unpack in your web root.
2. Chown to root:root
3. Chown FXIME to user and group of apache process
4. Create and folders in /var/local and make them writeable for to the webserver (chown to user and group of apache process)
5. Protect web access to subfolders

Run config/conf.pl:

1. Set server name to web.de
2. Set IMAP server to URL imap.web.de, Port
3. Set SMTP server to URL smtp.web.de, Port
4. Optionally set auto logout time
5. Map email folders: Draft→Entwurf, Sent→Gesendet Trash→Papierkorb

All settings are stored in config/config.php. More Info here

Everything done in Options

• Display Preferences → Custom Stylesheet: sans-10 looks good
• And I like Display Preferences → Theme: Penguin
• Optionally set Folder Preferences → Auto Refresh Folder List: never if you want to avoid page reloads