The internet's oldest surviving application. Ray Tomlinson sent the first one in 1971 between two ARPANET computers. He chose the @-symbol as the user@host separator.
Stack: SMTP (sending, port 25/587/465), IMAP (reading from server, 143/993), POP3 (download, 110/995). MTA = Mail Transfer Agent (Postfix, Exim), MUA = Mail User Agent (Thunderbird, Outlook). Anti-spam stack: SPF (who is allowed to send from this domain), DKIM (signatures), DMARC (policy). Modern reality: Gmail + Microsoft 365 dominate, deliverability from self-hosted mail server is nearly impossible.