1. Open Mac Mail preferences
First up, let’s open Mac Mail, then top left, click File followed by Preferences
2. Create a new placeholder signature
Along the top we see General, Accounts, Junk… etc. We want Signatures.
Click the + under column 2. You will a new signature #1 appear.
Feel free to name it anything you like. In the 3rd column, type a placeholder – the word “HERE” works.
Click and drag the new signature from the middle column and drop it on the email accounts in the left column you wish to use your new signature. Don’t worry, we will get to replacing the “HERE” with your html shortly.
You can now close this window with the red x top left and close Mac Mail with Command + Q on the keyboard.
3. Navigate to placeholder
Now we replace the newly created signature with your html code.
Open a new Finder window.
In the menu bar click Go. Now, this is nifty. Hold the Option Key on your keyboard, you see that new menu option that’s popped up Library – let’s click that.
A new finder window will pop up and you’ll be sitting in Library. Within this folder we want Mail / V2 / MailData / Signatures containing .mailsignature files. If there is more than one, we need to find the most recent.
This will be easier in finder list view, hit Command + 2 and now you can see a Date Modified column. We’re looking for the most recent. Now Control + Click (or right click), Open With, TextEdit.
A giveaway will be seeing ‘HERE’ within the code.
4. Replace the code
Have your html signature code ready by opening the file in TextEdit the same way as above.
Now we simply replace everything below the Mime-Version line with our html email signature and hit Command + S to save. If it asks you to unlock the file, click Unlock.
5. Lock it up
Click the little downwards facing arrow on the top bar, and select Locked, then Command + Q again to quit. You can also close the finder window now.
6. Send away
Now when you open Mail, you will have a fancy new html signature.
Compose a new email and you’ll see a signature drop down on the right hand side. Select your new signature.
Within preferences, you also have the option to change the default email signature for each mail account.
7. That's it
Enjoy your snazzy new html email signature.