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: cd squirrelmail; chown -R root:root
3. Chown FXIME to user and group of apache process
4. Create squirrelmail/data/ and squirrelmail/attach/ 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 143
3. Set SMTP server to URL smtp.web.de, Port 25
4. Map email folders: Draft→Entwurf, Sent→Gesendet Trash→Papierkorb
5. Optionally install the TimeoutUser plugin

All settings are stored in config/config.php. This is an excerpt with the important settings:

$squirrelmail_default_language = 'en_US';
$default_charset        = 'utf-8';
$domain                 = 'web.de';
$imapServerAddress      = 'imap.web.de';
$imapPort               = 143;
$useSendmail            = false;
$smtpServerAddress      = 'smtp.web.de';
$smtpPort               = 25;
$pop_before_smtp        = false;
$imap_server_type       = 'other';
$invert_time            = false;
$trash_folder           = 'Papierkorb';
$sent_folder            = 'Gesendet';
$draft_folder           = 'Entwurf';
$data_dir               = '/var/local/squirrelmail/data/';
$attachment_dir         = '/var/local/squirrelmail/attach/';
$smtp_auth_mech         = 'login';
$imap_auth_mech         = 'login';
$use_imap_tls           = false;
$use_smtp_tls           = false;

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