We use SendGrid. I first used them on a project I hosted on GAE which needed transaction emails and GAE recommended them (there was a free tier for GAE Apps).

We use SendGrid. I first used them on a project I hosted on GAE which needed transaction emails and GAE recommended them (there was a free tier for GAE Apps). We are planning on switching sometime next year as we are no longer that happy with them - documentation is getting more complicated, costs have increased, support is no longer as great as before.