How to change/customize mail signature in Mountain Lion?

Solution 1:

Quit Mail, edit the .mailsignature file with a text editor changing the HTML to whatever you like then right click the file in Finder and select "Get Info" then tick the "Locked" checkbox. This prevents Mail from resetting the file to the original version and it uses your version.

Solution 2:

• open Mail
• create a new signature
• open this folder /Users/you/Library/Mail/V2/MailData/Signatures/ • open the latest *.mailsignature file with a text editor
• replace everything under this code with your custom html (replace iso-8859-1 with utf-8 for umlaut support)

Content-Transfer-Encoding: quoted-printable
Content-Type: text/html;
    charset=iso-8859-1
Message-Id: <DDFB0C4D-5555-475C-9B54-115039EF5FE0@Speedport_W_722V_Typ_B>
Mime-Version: 1.0 (Mac OS X Mail 6.0 \(1486\))

• save the file and open the Get Info Dialogue and check Locked, so that it doesn't get overwritten by Mail

Solution 3:

I wrote a Ruby script to batch convert .webarchive to .mailsignature files: https://github.com/paulschreiber/misc/blob/master/ruby/webarchive2mailsignature.rb