Postfix is a free and open-source mail transfer agent (MTA) that routes and delivers electronic mail, intended as an alternative to the widely used Sendmail MTA. As an SMTP server, Postfix implements a first layer of defense against spambots and malware. Administrators can combine Postfix with other software that provides spam/virus filtering, message-store access, or complex SMTP-level access-policies. As an SMTP client, Postfix implements a high-performance parallelized mail-delivery engine. Postfix is often combined with mailing-list software.