I'm looking for a mail client for macOS that supports creating you own HTML mail templates for individual mail accounts. As an example, let's say I start writing a new email and it opens up to an empty email with an HTML template already loaded "around" and the cursor positioned in the template for me to type in.
Currently I am using the the SmartTemplate4 add-on for Postbox 5 that lets me do exactly that. However, since Postbox dropped add-on support for Postbox 6 and up, I can't use it with the new versions.
I know, that there are a few workarounds for this, for example in Apple Mail, you can create your HTML mail as an .html file, open it up in the browser, select everything and then copy it back into your new email, but this seems neither intuitive nor feasable for everyday use, especially if you are writing a ton of emails.
Any suggestions would be appreciated.