Mail app and templates for emails

You can do what you ask, but it requires a Mailbox > Rebuild operation. That asks Mail to scan the ~/Library/Mail directory you found and rebuild its email header database from the files it finds there. I doubt you'd find that acceptable.

I'm afraid Gerry's comment above is the right way to go about this: set up a test email account somewhere, use one of the many PHP-to-SMTP methods you can find online, and point Apple Mail at that email account.

Perhaps the simplest, most Apple-like way to achieve that end is with OS X Server. It gives you a configuration UI for the Postfix SMTP mail server built into OS X. Setup through the Server GUI is about as easy as you can hope for. You can create a private user account, and be assured that your test emails won't ever even leave the machine. As cheap as OS X Server is these days, it's pretty much a no-brainer.

If you absolutely have to do this for no cost, you can configure Postfix by hand. The configuration GUI you get with OS X Server is helpful, but not strictly necessary.

Look in the Account settings in Apple Mail and look at the Outgoing Mail Server (SMTP) setting and see if it matches the correct mail service. (i.e. is it using the Gmail smtp server if it is a gmail account. Also, you can use the drop down list and select Edit SMTP Server List to select that particular SMTP server's settings. For Gmail it would be smtp.gmail.com and mine is set to use the default ports, has Authentication as password, and has my login credentials.

Hope this helps.