In case you cannot choose signature in Mac Mail app, you can create your personalized signatures and add them to your emails as per your wish. It is not that only one signature can be used, but you can create multiple signatures. You can also create signatures for a particular email account in the Mail app, and can also choose the default signature for that account. In case you want to change the signature for that email account, then you can do it while composing your email.
Sometimes, what happens is, after an OS update or upgrade, when you launch the Mail app and compose your email, the default signature for that email account might not be included in the compose window. So, when you go to the Signature pop-up menu to set a signature, there won’t be any signature listed. Only the None and Edit options will be available.
Also, when you select the Edit option, the Signature preference window will be opened. In this window, the Choose Signature menu will be greyed out, and you cannot choose signature in Mac Mail. Follow the methods explained here to solve the issue and choose your signature in Mac Mail.
If you had created your signatures under the All Signatures option, then you might face this issue. To choose a signature for your email account, you have to add the signatures to that account. The steps for this method are given below.
If you are still not able to choose a signature, then go to method 2.
In Mac Mail, the signatures are associated with the Signatures ByAccount.plist file. Moving this file to Trash would solve cannot choose signature in Mac Mail issue. The steps are: