I just made this PHP class. It's extremely simple and easy to use for anyone who knows the slightest bit about PHP - I hope.
I needed to send lots of emails, but have it look like I'd sent each one individually. I had a search, and there were ZERO online services to help me do it - which surprised me a little - and lots of code snippets to help you do it, but nothing really comprehensive.
This is why I've written this. Feel free to use it. I just used it and it worked a treat.