Squirrelmail - end users
If you're using SquirrelMail to read and write mails, but someone else manages the SquirrelMail installation you're using, you're an end user.
Squirrelmail - administrators
If you're responsible for managing a SquirrelMail installation, you're an administrator. Persons in this category are usually responsible for installing, configuring, patching, and upgrading the SquirrelMail installation. Most administrators also have access to the other parts of the mail system, such as the user database.