Ask YC: Reliable and quick smtp relay service

I have a site which is ready to be launched in couple of weeks. Unfortunately we realized that the service we were using until now to deliver our notification emails is not passing performance bar.

We need to find an smtp service which we can use to deliver emails after account creation, password resets and account activity.

Any recommendations ? Here are the requirements we are working with -

- Need to integrate with Rails

- Need to send it using unlimited from addresses or allow us flexibility to add reply to address in header

- We prefer somebody from US.