I don't use Mac Mail anymore, but I did find a few things. Some people have found that there Smart Mail Boxes are in the Sidebar but are hidden (sort of). If you see On My Mac and hover your cursor over that to the right of it, the word 'Show' will appear, and if you click on that, then your Smart Mail Boxes
could be under that ??
If not, try the solution in this
Apple Discussion Thread in the 2nd Post by
hoosierchick2012
As to the Signature, you might have to go into and grab the AllSignature.plist
Make sure Mail has been QUIT . . . . In Finder "Go To Folder" under Finder>Go>Go To Folder Click on it and a window will drop down. Paste in :
Code:
~/Library/Mail/V2/MailData/Signatures
and look for the AllSignatures.plist and drag it to your Desktop. Open Mail, and then in the Preferences>Signatures see if you can now add your Signatures and if so add one, and quit Mail, open it again and see if its still there. If it is, assign it to a email address and then you should be all good.
HTH