To have a signature on all of your outgoing emails, you can add one in SquirrelMail in the Personal Information section.  First, log into your SquirrelMail and click on Options at the top of the screen.  This will bring up the following window:

Click on "Personal Information," and on the next screen you will be able to insert your Signature:

Once you are done hit "Submit" on the bottom right and now all of your outgoing emails will have your new signature.