A fourth part for the client side is the MUA - Mail User Agent. It is also called (although not standardized by IETF) MRA - Mail Retrieval Agent if the client is capable of receiving mail in addition to being able to compose it.

They support the SMTP protocol for sending mails to mailboxes, and the POP3, IMAP and JMAP protocols for sending mails to email clients upon request.

Google mail server addresses

MSA:

gmail-smtp-in.google.com
smtp.google.com - CNAME to gmail-smtp-msa.l.google.com
gmail-smtp-msa.l.google.com - normal Gmail/GWorkspace mail; Port 465 (SSL)/Port 587 (TLS)
aspmx.l.google.com - SMTP Restricted only to gmail/your GWorkspace organization; Port 25 (No TLS required); add SPF of domain to GAdmin Console

MTA

smtp-relay.google.com - SMTP Relay

POP3/IMAP Servers:

imap.google.com - CNAME to gmail-imap.l.google.com
gmail-imap.l.google.com - IMAP Server
pop.google.com (may be a CNAME too)

Mail Server software

Components

Components include: Mail User Agent, Mail Transfer Agent, Mail Delivery Agent, Anti-spam and Antivirus software

MUA (SMTP/POP3/IMAP/JMAP Client)

In UNIX philosophy, the MUA and the MTA are decoupled. In other cases, MUA usually has a send-only MTA.

BSD mailx
heirloom-mailx
S-mailx
GNU mailutils

Standard Shell:

/usr/bin/mail

MSA / MTA / MDA (SMTP Client/Server)

Full MTA (Relay Capable):

Sendmail
Postfix
Exim

Minimal MTA (Send Only):

msmtp

Standard Shell:

/usr/sbin/sendmail

POP3/IMAP/JMAP Server

Dovecot

Full Servers

Mail-in-a-box
iRedMail

MySQL/MariaDB - Virtual Mailboxes

Mail terminology: MTA, MUA, MSA, MDA, SMTP, DKIM, SPF, DMARC | Sysadmin

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