Client and server components
MUA (Mail User Agent)
Client application that allows receiving and sending emails. It can be a desktop application such as Microsoft Outlook/Thunderbird/… or web-based such as Gmail/Hotmail/… (the latter is also called Webmail).
MSA (Mail Submission Agent)
A server program that receives mail from an MUA, checks for any errors, and transfers it (with SMTP) to the MTA hosted on the same server.
MTA (Mail Transfer Agent)
A server application that receives mail from the MSA, or from another MTA. It will find (through name servers and the DNS) the MX record from the recipient domain’s DNS zone in order to know how to transfer the mail. It then transfers the mail (with SMTP) to another MTA (which is known as SMTP relaying) or, if the recipient’s server has been reached, to the MDA.
Examples of MTAs are Postfix, Exim, , …
MDA (Mail Delivery Agent)
A server program that receives mail from the server’s MTA, and stores it into the mailbox. MDA is also known as LDA (Local Delivery Agent).
An example is Dovecot, which is mainly a POP3 and IMAP server allowing an MUA to retrieve mail, but also includes an MDA which takes mail from an MTA and delivers it to the server’s mailbox.
The server’s mail storage. Maildir is a way of storing email messages. It is usually preferable over mbox.
Protocol used by MUAs to send emails to an MSA. The recommended SMTP port for sending mail (from an MUA to an MSA) is the port 587, which uses TLS encryption.
Protocols used by MUAs to retrieve emails from a server mailbox. POP3 deletes the email messages from the server after they have been downloaded. IMAP is usually preferable as it maintains all email messages on the server, permitting management of a mailbox by multiple email clients.
MX (Mail Exchanger) record
A Mail Exchanger (MX) record in the DNS specifies which server is responsible for accepting email addresses on behalf of a domain. The host name from the MX record must map to one or more address record (A or AAAA) in the DNS, and must not point to any CNAME records.