It looks real good.
but I don't get the catch: is it free service ?
is there a guarantee on the time between the user posting the form and me receiving the email ?
what if your site is down ?
Excellent questions. It is free unless you want to use custom fields. The emails go into a queue which is listened for and processed when items are added. I have 3 that listeners that run simultaneously to handle email sending.
If the site goes down then the emails are lost. I'm still working on a solution of multi region load balancing with high availability to ensure this doesn't happen.
FYI, there's no guarantee of delivery, ever, where e-mails are involved. Guaranteed delivery is not one of its features. Best-effort store-and-forward, yes; for guaranteed delivery, you need a different tool.