My webapp needs to send customized messages to members and I'm wondering if there is an inexpensive and easy to use web service that would meet my needs. The types of mail I will be sending include:

• New account activation email (sent ASAP)
• Daily status report of user's account (sent anytime)
• Event reminders (sent at specific time)

Specifically, I would like the following features:

• RESTful API to add an email message into the send queue
• A way to add a priority to each message (account signup activations should be sent immediately, while a daily status report could be sent anytime each day)
• They manage the sending of mail and the processing of bounces
• Possibly, they manage opt-out/opt-in features
• They offer features such as DKIM, VERP, etc.
• If their service determines an address is undeliverable (via VERP or other means, a user unsubscribes, etc), they make a RESTful call to my web service to notify me
• Nice if they had some reporting features, WebBugs, link tracking, etc.

What I am NOT looking for is an email marketing service that caters only to sending out copies of the same mail to masses of recipients. I need to send out unique and custom messages to individuals. I had a chat with MailChimp about using their services for sending these types of messages and they said their service does not support customized emails per recipient.

Edit: I just discovered a service called JangoSMTP that appears to meet many of my requirements. It provides an API for sending mail, supports DKIM, bounce management, and even feedback loops. Unfortunately, their idea of inexpensive doesn't mesh with mine, as it would cost $180/mo to send a single daily message to my 1000 users.