Linux email services are generally categorized into Mail Transfer Agents (MTAs) that send mail, Mail Delivery Agents (MDAs) that receive mail, and full-stack, all-in-one mail server solutions.
Core Email Components (Linux Infrastructure)
Linux systems rely on these specialized services for the "Unix way" of handling mail:
Postfix (MTA): The most common Mail Transfer Agent, responsible for transferring email messages between servers.
Dovecot (MDA/IMAP/POP3): A secure Mail Delivery Agent that handles incoming mail and provides IMAP/POP3 access for client retrieval.
Sendmail (MTA): The classic, foundational mail transport agent.
Procmail/Maildrop (MDA): Mail delivery agents that filter and sort email into mailboxes.
Fetchmail: Retrieves mail from remote servers
Essential Email Protocols
SMTP (Simple Mail Transfer Protocol): Standard for sending mail (Port 25 or 587).
IMAP (Internet Message Access Protocol): Synchronizes emails across multiple devices.
POP3 (Post Office Protocol 3): Downloads emails, often removing them from the server.
Security and DNS Records
SPF (Sender Policy Framework): Identifies authorized IP addresses for sending mail.
DKIM (DomainKeys Identified Mail): Adds a digital signature to emails.
DMARC: Tells servers what to do if SPF or DKIM fails.
MX Record: Directs mail to the correct server