Why do mail signature is being set back if I do not lock file?
I have used that way for creating an HTML signature in mail:


Create a new signature with a preferred text
Full close mail app
Go to:  Library ▸ Mail ▸ V2 ▸ MailData
Open all files with .mailsignature extension until you find the text that you have writed in the first passage
Copy that file in to your desktop
Unlock it
Put your HTML code
Save and lock again that file
in Library ▸ Mail ▸ V2 ▸ MailData delete the old file .mailsignature
Copy your new .mailsingaturefile with your HTML code


If you don't do that passage mail will replace all .mailsingature files that is no locked or not correspond whit his signature name code.