As Chuck indicates, different email clients (Outlook, Outlook Express, Thunderbird, Windows Mail, etc) will have slightly different ways of handling signatures. This text is from the Outlook Express help files:
"To add a signature to outgoing messages
On the Tools menu, click Options, and then click the Signatures tab.
To create a signature, click the New button and then either enter text in the Edit Signature box or click File, and then find the text or HTML file you'd like to use.
Select the Add signatures to all outgoing messages check box.
Tips
To use different signatures for different accounts, in the Signatures area, select the signature, click the Advanced button, and then select the account you want to use the signature with.
To use a signature on individual messages only, make sure to clear the Add signatures to all outgoing messages check box. When you compose the message, click the Insert menu, point to Signatures, and then click the signature you want.".
Chris