SMTP

Mail Routing Architecture

SMTP (Simple Mail Transfer Protocol) is the standard protocol for sending and relaying email between mail servers. When you press Send, your email client submits the message to your outgoing mail server via SMTP. That server then looks up the recipient domain's MX RecordMail Exchange record. A DNS record that specifies the mail server responsible for receiving email for a domain, along with a priority value that determines the order in which servers are tried. in DNSDomain Name System. The hierarchical, distributed naming system that translates human-readable domain names (e.g., example.com) into IP addresses (e.g., 93.184.216.34). Often called the "phonebook of the internet." to find the destination mail server and delivers the message directly via SMTP. Each hop in this relay chain is an SMTP transaction.

Key Ports

SMTP uses three ports with distinct roles. Port 25 is for server-to-server relay between mail servers (MTA to MTA) and is typically blocked by ISPInternet Service Provider. A company that provides internet access to consumers and businesses, assigning public IP addresses and routing traffic to the wider internet. Examples include Comcast, AT&T, and SK Broadband.s for residential connections to prevent spam. Port 587 (submission) is for authenticated client-to-server submission — mail clients use this with username and password. Port 465 is SMTP over SSL/TLSSecure Sockets Layer / Transport Layer Security. Cryptographic protocols that provide encrypted, authenticated communication over a network. SSL is deprecated; modern implementations use TLS 1.2 or TLS 1.3. (SMTPS), used for implicit TLS from the start of the connection.

Spam Prevention Mechanisms

Plain SMTP has no built-in sender authentication, making it trivial to forge the From address. Modern mail infrastructure layers several DNSDomain Name System. The hierarchical, distributed naming system that translates human-readable domain names (e.g., example.com) into IP addresses (e.g., 93.184.216.34). Often called the "phonebook of the internet."-based authentication protocols on top: SPF (TXT RecordA DNS record that holds arbitrary text data. Commonly used for email authentication (SPF, DKIM, DMARC), domain ownership verification, and other machine-readable metadata. listing authorized sending IPs), DKIM (cryptographic signature in headers verified via a TXT RecordA DNS record that holds arbitrary text data. Commonly used for email authentication (SPF, DKIM, DMARC), domain ownership verification, and other machine-readable metadata.), and DMARC (policy TXT RecordA DNS record that holds arbitrary text data. Commonly used for email authentication (SPF, DKIM, DMARC), domain ownership verification, and other machine-readable metadata. instructing receivers what to do with messages that fail SPF or DKIM). Together these form the backbone of modern email deliverability and anti-spoofing. Use IP Blacklist Check to verify a mail server's IP is not listed on spam blocklists.