FYI - the confirmation email ended up in my spam folder. I'm not sure if that's gmail's default behavior or whether I've encouraged it to be a little too agressive, something to watch out for though.
Do you have any suggestions what I could improve to make these emails pass?
You'll find a continuum of options for e-mail sending services. I'd pick the one that's most trusted within reason, not necessarily the "best deal." To me, it's an image thing: every time your e-mail ends in a Spam folder you let that user down a little. If they do find it, you've still disappointed them a little.