Is there any webservice (API) that can receive emails and process their attachements to deliver them into my app?
Based on your comment to Messa it sounds like you don't want an API at all. You're looking for an external service to "read" and process emails in a quite specific way.
Have you looked at implementing something to retrieve the emails and process them yourself? Depending on the language/tools you're using I'd be surprised if this was very difficult. (I've done this sort of thing in several languages before within little problem.)
If you're using an external service/API you'd have to give it credentials to access your email and S3 accounts. You probably want to be wary of this if security is any kind of requirement for you.
Sendgrid.com has a Parse API that could do the job in fact : http://wiki.sendgrid.com/doku.php?id=parse_api
This is great!