Components of a mail server
The following components are required to create a complete delivery chain.
MUA (Mail User Agent)
Client/program capable of sending emails to a mail server; while also capable of fetching emails from a mail server for presenting them to the end user.
MTA (Mail Transfer Agent)
Software dedicated to accepting submitted emails, then forwarding them.
If the receiving MTA is responsible for the FQDN the email is sent to, then an MTA is to forward that email to an MDA.
Otherwise, it is to transfer to another MTA, “closer” to the email’s final destination.
MDA (Mail Delivery Agent)
Accepts emails from an MTA and dropping them into their recipients mailboxes.
Schematic view
Sending an email: MUA ----> MTA ----> (MTA relays) ----> MDA
Fetching an email: MUA <--------------------------------- MDA