I cannot get my signature to appear on my sent emails at all, although it appears on my screen while writing an email. After going to the Apple Store, the Genius couldn’t resolve the issue after deleting all of my signatures.

Let’s start fresh by copying and pasting your existing signature somewhere else, and then deleting all of the signatures in the All Signatures area of Mail > Preferences > Signatures.

Click an email account in the signatures listing to set a default signature for that account.

After doing that, click All Signatures, and then create a new signature by clicking the plus (+)  button. Notice that your email accounts are also listed in the same area. You’ll drag the name of each of the signatures from the All Signatures section to the email account or accounts that should have access to that signature.
 
To set a default signature for an email account, select the email account you just dragged the signature to, and then choose the signature from the Choose Signature pop-up menu. You can also check the box to “Place signature above quoted text.” After doing this, your signature will automatically appear in the New Message window when you create an email with the specified account.

In the New Message window, you can also choose between your different signatures.

If you have more than one signature for the email account you’re sending a message from, then you’ll get the Signature option in the New Message window to choose which signature you want to include.

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