MTA: “Mail Transfer Agent”. MTAs relay mail from point to point until it can be delivered. E-mail is submitted by other server using the SMTP protocol to TCP port 25, or by local clients with the /usr/bin/sendmail program. If the MTA is the final destination, the message is passed to the MDA. If not, it looks up the next MTA in DNS using MX records and relays it there using SMTP.
MDA: “Mail Delivery Agent”. The MDA delivers mail to the recipient’s local message store(by default /var/spool/mail/user). Postfix provides its own MDA to deliver to the default local file-based message store, /usr/libexec/postfix/local.
MUA: “Mail User Agent”. Clients used to send e-mail and read e-mail in the user’s message store.
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