I use the same signature for all my emails, but this is only because I can barely think of one signature that is only mildly silly. Normally, using different signatures for work (boringly useful) and private (annoyingly funny) accounts, for example, makes perfect sense.

It makes sense, and Mac OS X Mail can put the correct signature for an account in your emails automatically. But firs you have to specify which signature you want to be the default for each account.

Set a Default Signature for an Account in Mac OS X Mail

To define the default signature for an email account in Mac OS X Mail:

• Select Mail | Preferences... from the menu.
• Go to the Signatures tab.
• Highlight the desired account.
• Select the desired signature under Choose Signature:.
  • To create a new signature for an account, press the + button.
  • To use a signature created for another account, select All Signatures and drag the desired signature to the account.