OpenProtect is an anti-spam server that filters spam and blocks viruses, phishing, and other malware. Acts as an SMTP gateway and integrates with any email server like Postfix, Sendmail, qmail, Exim, Microsoft Exchange, Lotus Domino, Zimbra, etc.
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.