With this solution I still have to setup an intermediate mail server solution to first receive the emails from users, and then forward it along. Can't you take that step away from me?
Other providers (mailgun, sendgrid, etc) let me do that and let users send emails directly to my inbound mailboxes (which are then POSTed to my endpoint).
Also, this excerpt from the documentation lowers my confidence that you know what you're doing:
"Rather than leaving this information in plaintext, we recommend using SHA1 or MD5 to encrypt the information after the + sign and include the encrypted value after the +. This will avoid unintentionally exposing any sensitive information."