After the latest iOS update on my iMac, I also lost all signatures from my mail preferences. When I recreated them from old sent emails, I could not get them to show up as available choices when composing a new email.
My problem, solved by a good Apple support tech, was that I had multiple internet accounts for the same email address from my mail provider (Comcast), one for Notes and one for Email. They were not named clearly enough so in the Mail preferences, these accounts showed similarly on the left when in the Signature editing panel. I had created my signatures for the Notes account, which will not show when composing an email. When I recreated the signatures in the correct email account, all the signatures showed up as expected.
I then renamed the internet Notes account in System Preferences to include "Notes" in the account name to avoid making the same mistake again. You can also match your account name in the Mail Inbox on the left hand side to the account name in System Preferences under Internet Accounts to ensure you get your signatures attached to the correct account.
This will not apply to everyone's missing email signatures, but may help others like me. Below shows the
account names after renaming to include "Notes" to clearly identify which ones were for Notes only. Prior to renaming, they were almost identical. Thanks Apple Tech Support.
