mod_sendmail is a restful HTTP to SMTP gateway for Apache httpd. It lets you send email by using a restful HTTP interface, as part of the Mail Service project. mod_sendmail is an Apache httpd module that gates HTTP requests to a sendmail compatible MTA binary like Postfix.
mod_processdsn for Apache httpd exposes delivery status notifications recorded within an SQL database via a restful HTTP interface. The delivery status notifications are expected to have been recorded by the processdsn tool as part of the Mail Service project.