MTA
also: Mail Transfer Agent, SMTP server
MTA (Mail Transfer Agent) is a server program that sends, receives, and routes email messages between servers using SMTP protocol.
An MTA is the core component of email infrastructure responsible for accepting outgoing messages from users or applications and delivering them to recipient mail servers across the internet. It handles the technical details of SMTP (Simple Mail Transfer Protocol) communication, including DNS lookups to find recipient mail servers, message queuing, and retry logic for failed deliveries.
Common MTAs in Linux include Postfix, Sendmail, Exim, and OpenSMTPD. For example, when you send an email from your application, it typically connects to the local MTA on port 25, which then takes responsibility for getting that message to the destination mail server.
MTAs are distinct from MUAs (Mail User Agents) like Thunderbird or Gmail, which are client programs that help users read and compose emails, and from MDAs (Mail Delivery Agents) like Procmail, which handle final local delivery to user mailboxes.