Creating an email signature
The first thing you will need to do before adding the HTML is to create a signature. This can be done by opening the ‘Mail’ app and then select the ‘Mail’ tab at the top, then Preferences.
The ‘Preferences’ box will then appear. Select the ‘Signature’ tab, and click on the ‘+’ button to add a signature, name it something appropriate. Don’t worry about the contents at this point as we will be changing that later.
Adding HTML to your new signature
At this point, you will want to close the ‘Mail app (be sure to completely quit out of it) and open a finder window. Once open, select the ‘Go’ tab on the top bar and click on ‘Library’, you may have to hold down the ‘Option’ button on your keyboard for the ‘Library’ option to appear.
When the folder opens you will want to navigate to Library > Mail > V6 (or the ‘V’ folder with the highest number) > Mail Data > Signatures – this is the folder where your email signatures are stored. You should see a file in this folder that looks similar to the one below.
Open this file with a text editor of your choice or the ‘TextEdit’ app on your Mac. You will then see something that looks like the example below. You will want to delete the body section and replace it with the desired HTML for your signature.
When you have added the new HTML you will need to lock the file. You can do this on the ‘TextEdit’ app by selecting the drop-down arrow.
Then tick the ‘Locked’ box.
Using your new signature
You can now open the ‘Mail’ app again. Navigate to where you first created your signature, click on the signature that you created, an example of your signature should appear in the box on the right. Now drag and drop the signature into the account of your choice which is listed on the left.
You can now open a new email and add the signature to it.