SquirrelMail is available to download from http://www.squirrelmail.org/
Before installing, make sure you have Apache, PHP and Dovecot installed ok.
Download and unpack SquirrelMail:-
wget http://downloads.sourceforge.net/sourceforge/squirrelmail/squirrelmail-1.4.13.tar.gz gunzip squirrelmail-1.4.13.tar.gz tar -xvf squirrelmail-1.4.13.tar
You now have all the SquirrelMail files in the directory called squirrelmail-1.4.13 so you need to move them all into the webspace that you wish to run SquirrelMail from. This maybe a suitable 'webmail.yourdomain.co.uk' virtualhost directory?
Inside your DocumentRoot webspace directory, move the entire SquirrelMail distribution,
mv /root/squirrelmail-1.4.13/* .
You now need to create the attachments and data-dir directories, which are required for SquirrelMail. I generally put these in a /var/squirrelmail/ directory, but they can go in your /var/mail/ directory, along with the mail files for your users.
Make sure the two directories are owned by the same 'user' that Apache runs as:-
chown apache:apache /var/mail/attachments chown apache:apache /var/mail/data-dir
From the SqirrelMail directory, run config/conf.pl to setup the relevant options for your installation.
Option 1 allows you to set the various organisation preferences for your SquirrelMail installation. From here you can select the name and logo for your organisation.
Option 2 is important to get correct, otherwise you may find SquirrelMail doesn't function correctly.
The Domain should be set to the domain your emails come from. eg. if you're emailing people as email@example.com then set the domain to "yourdomain.com"
Leave the Invert Time alone, unless you know what you're doing.
Set the Sendmail or SMTP option to Sendmail which will allow your server to send mail directly rather than connecting to an SMTP server. However this, and other, settings can be customised to individual requirements.
Set the IMAP settings accordingly to your setup. This will depend upon whether you're running your mail server on the same physical server as your webmail system. If not, you will need to point the IMAP stuff at your IMAP server (ie. mail server running Dovecot)
In normal usage you shouldn't need to change anything in here, but you may want to examine the settings to see what options are available.
Again important to get right, especially if you've changed the default names for the attachments and data directories mentioned above. The main settings to get right are options 1 and 2 - the location of the Data and Attachment directories. The rest can be left alone, unless you know what you're doing.
The user address books and preferences are stored in or possibly /var/lib/squirrelmail/prefs. The address book is suffixed .abook, the options are suffixed .pref and any signature is suffixed .sig.
One benefit of SquirrelMail is that it can handle multiple plugins to enhance it's usability and add extra features. These are described on a separate Plugins page.