The webmail portion of the project was started by Nathan and Luke Ehresman in 1999 and is written in PHP. SquirrelMail can be employed in conjunction with a LAMP "stack", and any other operating systems that support PHP are supported as well. The web server needs access to the IMAP server hosting the email and to an SMTP server to be able to send mails.
SquirrelMail webmail outputs valid HTML 4.0 for its presentation, making it compatible with a majority of current web browsers. SquirrelMail webmail uses a plugin architecture to accommodate additional features around the core application, and over 200 plugins are available on the SquirrelMail website.
The SquirrelMail IMAP proxy server product was created in 2002 by Dave McMurtrie while at the University of Pittsburgh (where it was named "up-imapproxy", although it has become more commonly known as "imapproxy") and adopted by the SquirrelMail team in 2010. It is written in C and is primarily made to provide stateful connections for stateless webmail client software to an IMAP server, thus avoiding new IMAP logins for every client action and in some cases significantly improving webmail performance.
SquirrelMail webmail is available for any platform supporting PHP. Most commonly used platforms include Linux, FreeBSD, OS X and the server variants of Microsoft Windows. SquirrelMail IMAP Proxy compiles on most flavors of Unix, and can generally be used on the same platforms as the webmail product can be with the exception of Microsoft Windows, unless used in a Cygwin or similar environment. Apple shipped SquirrelMail as their supported web mail solution in Mac OS X Server.
The SquirrelMail webmail client itself is a complete webmail system, but extra features are available in the form of plugins. There are over 200 third-party plugins available for download from the SquirrelMail website and SquirrelMail ships with several "standard" or "core" plugins.