Which of the following are commonly used Mail Transfer Agent (MTA) applications? (Select THREE).

Postfix is a popular open-source Mail Transfer Agent (MTA) known for its security and ease of configuration, widely used on Unix-like systems for routing and delivering email.

Procmail is a Mail Delivery Agent (MDA), not an MTA; it is used for local email delivery and filtering, not transferring mail between servers.

Sendmail is a historic and powerful MTA, one of the earliest and most widely used email routing programs on the internet, handling mail transfer between systems.

Exim is a flexible and configurable MTA developed for Unix-like systems, particularly popular in academic environments and with many internet service providers for email routing.

`smtpd` is commonly the name of the daemon process for an MTA (e.g., `sendmail` or `postfix` running in daemon mode), but it is not an MTA application name itself.