High Sierra Mail Preferences Missing When I go to "Preferences", a window named "Signatures" appears, which is empty. All my many previously saved Signatures are gone, and there is no way to access Mail Preferences.
Hopefully you have a Time Machine (or other) backup that you can restore the files, which are located in 

~/Library/Mail/V5/MailData/Signatures 

and the files seem to be called by some sort of PID string ending with ".mailsignature"

Restore those files from backup to get your signatures back.