found this some time ago on here

Sending HTML email with Apple Mail

Hey guys,

I recently released Email Pack, a project file based on Email stacks.
Someone asked me if it is possible to use Email Pack templates to send emails with Apple Mail, well the answer is YES. Just follow the following procedure, it seems long but it takes less than 1 minute.

Create a new mail in Apple Mail app, write just the word placeholder then hit cmd + s to save a draft of the email.
Find your email draft on the left panel of Apple Mail app and drag it from Apple Mail onto your Desktop.
Open the email file with any text editor you like (e.g. TextEdit)
Search for the word placeholder you wrote earlier. Replace ONLY the placeholder word with your entire HTML code generated by Email stack, then hit cmd + s to save the file.

Do not delete anything else! Don't replace the current HTML tag just the word placeholder
In the Apple Mail app select the menu File --> Import Mail Boxes
Select Apple Mail & import the edited file.
Now you will see your mail on the left panel (probably at the bottom) of Apple Mail app, most likely inside an Imported mailbox folder.
Don't open it yet, and drag it from that folder into your Draft mailbox.
Finally, open the mail by double-click (or right-click + Open), fill in To, CC, Subject, etc. You can also edit text... and then send your pure HTML mail!

I tested some templates and it seems to work well 🎉

How to get email design from Rapid Eaver to Apple mail so I can send it out?