Rich Text HTML Signature for Apple Mail on Mountain Lion
works a little different than a signature for Lion. If you don’t have your custom HTML ready yet, please go ahead and create one at this point. Please follow the article Rich Text HTML Signature for Apple Mail if you’d like some pointers or work with Lion instead. This is basically a recap of said article but the target files are at different locations. The procedure is roughly the same though.
Rich text signatures only apply to rich text emails. To create and attach html to your outgoing mail, switch your default mail format in Mail.app > Preferences > Composing > Message Format. In the drop down menu select Rich Text.
Now switch to Signatures and create a blank one. The content does not really matter as it is going to be replaced by our custom signature. This will only create the necessary file if you followed step 1.
Quit Apple Mail! This is important as Mail would overwrite the custom signature file otherwise.
Locate the file on you hard disk. It can be found at ~/Library/Mail/V2/MailData/Signatures/ and is usually a file named something like D879F2D5-CCA1-4BCA-AA5F-5EEB54995957.mailsignature but always ending in .mailsignature. This file is only created if you executed step 1. If you can not find the file double check you mail format settings.
Open your own signature markup file from your convenient location and paste it in between the <body> and </body> tags.
Lock you file so it is write protected and apple mail does not overwrite it as soon as you restart mail
Restart Apple Mail and check signature settings. You can select the new signature for a specific or all outgoing mail addresses in case you use multiple.
I have no idea why Apple decided to make it so hard to create full fledged rich text signature in mail. Probably because it is no well documented standard and support varies from mail client to mail client. But one can only guess. 09/28/2012