What is feature of SMTP?

Simple Mail Transfer Protocol (SMTP) is a standard protocol used for sending emails across the Internet. It defines a set of communication guidelines between mail servers to relay mail efficiently and reliably. Some key features of SMTP include:

What is feature of SMTP?

Client-server model
SMTP uses a client-server architecture where a mail client connects to a mail server to send emails. The client initiates a TCP connection with SMTP commands to the server on port 25. This allows centralized mail servers to handle sending emails for multiple clients.

Text-based protocol
SMTP is a text-based protocol with a defined set of ASCII text commands, reply codes and formatting that clients and servers must follow to communicate. This simplicity provides greater flexibility across platforms and languages.

Store and forward method
SMTP server does not instantly deliver email to the recipient. It accepts and queues up messages from clients then uses SMTP protocol rules to communicate with the next mail server until the email reaches the recipient mail server. This provides reliability in case of temporary connection failures with the next server.

SMTP handshake and HELO protocol
The initiating SMTP client needs to introduce itself with the SMTP HELO command identifying the sending server to begin the SMTP conversation. The receiving SMTP server acknowledges with a success response code. This handshake establishes initial connection.

Authentication protocols
SMTP supports authentication protocols like LOGIN, PLAIN etc. Using these authentication mechanisms, SMTP servers can securely validate user identity and verify if user has access permission to relay mails through the server.

Transport Layer Security (TLS)
SMTP communication can be encrypted using Transport Layer Security (TLS) which provides privacy between communicating SMTP hosts. It uses certificates and cryptography to provide confidentiality and integrity guarantees required for sensitive email transfers.

Error messaging and validation
SMTP mandates sending servers to validate email ids and content format. In case of errors, an appropriate SMTP error response code and message is sent back so the issues can be fixed by the sending client/server before reattempting message transfer through SMTP.

Message envelope and content
SMTP encapsulates the email content and recipient’s address into an envelope and delivers it across transport agents. Routers extract only the envelope to determine where to relay the message next without needing to process actual email content.

Key Takeaways:

  • SMTP is a standardized protocol enabling communication between mail servers for sending and relaying emails
  • Key facets like client-server model, use of TCP/IP, error handling make SMTP reliable and flexible
  • Security mechanisms like TLS and authentication protocols help keep sensitive email communications safe
  • SMTP is a backbone protocol fundamental to modern global email infrastructure allowing fast and efficient email transfers


In conclusion, SMTP plays a foundational role for email delivery by defining how messages should be correctly formatted, routed and securely transported between mail servers on the Internet. Its capabilities like queuing, error handling, authentication and TLS make SMTP robust and adaptable for sending emails globally across diverse networks and systems at large scale. Understanding SMTP helps build more compliant and optimized mail server ecosystems.


  1. What are the core SMTP extensions and proposed standards?
    Some major SMTP extensions and standards introduced over time are ESMTP, Delivery Status Notifications, Internationalization, SMTP-AUTH, SMTP over TLS, SMTPUTF8, etc. 
  2. How does SMTP server-to-server communication occur?
    SMTP mail transfer between servers happens based on MX records which specify the designated mail servers for a domain through DNS entries mapping domain names to server IPs. 
  3. What are some commonly used SMTP diagnostics tools?
    Some commonly used SMTP diagnostics tools are Telnet, OpenSSL, swaks, MailTester, smtp-user-enum, SMTP2GO, MailHog etc which help test SMTP connections and flows. 
  4. What are some of the default ports SMTP systems use?
    In addition to the standard SMTP port 25, variations like port 465 (SMTPS/SMTP+SSL) and port 587 (SMTP+STARTTLS submissions) are sometimes used with TLS encryption enabled. 
  5. What is the difference between SMTPS and SMTPS implicit/explicit?
    SMTPS supports implicit encryption natively. SMTP+STARTTLS establishes encryption later explicitly in the process through client command after initial connection. Implicit is more integrated while explicit allows more flexibility. 
  6. How can SMTP servers manage high volumes of mail traffic?
    Strategies like load balancing, clustering, sharding help partition and distribute traffic across servers. Caching improves performance. Rate limiting, greylisting handle floods during spikes. 
  7. What are some alternatives to running a full-featured, custom SMTP server?
    Some alternatives are using cloud-based SMTP services like Sendgrid, Mailgun, SES eliminating managing on-prem infra. Lightweight SMTP servers like Haraka, aisle8 have simpler needs. 
  8. How is SMTP performance optimized?
    Performance tuning steps involve – hardware sizing, connection pooling, concurrent connection limits, proper queue management, caching, running dedicated relay servers, enabling compression etc. 
  9. What are some key open source Java SMTP libraries?
    Some popular open source Java SMTP libraries are JavaMail, GreenMail, Apache Commons Email, SendGrid Dependency, Amazon SES SDK etc – offering reusable components for core mailing capabilities. 
  10. What are important aspects securing organization’s SMTP infrastructure?
    Critical security aspects are – TLS encryption, strong authentication and identity verification, access controls, monitoring, malware filtering, patching, host hardening, DMARC policies etc. 
  11. What are some alternatives to SMTP and differences?
    Some alternatives are XMPP for push messages, Direct Internet Message Encapsulation (DIME)/Blocks Extensible Exchange Protocol (BEEP) for replacing TCP, QUIC providing multiplexing over UDP etc. 
  12. How can SMTP systems automatically detect invalid recipients?
    Automatic checks before accepting messages include – validating domain exists against DNS, verifying user mailbox, checking against blocklists, greylisting to detect spammers etc using RCPT TO command. 
  13. What are some measures protecting SMTP against spammers?
    Anti-spam methods involve blacklists/RBLs, greylisting, spam filters, rate limiting, recipient verification, implementing CAPTCHAs, enforcing authentication, monitoring logs etc. 
  14. How can SMTP errors be classified and interpreted?
    Based on first SMTP response code digit – 2xx indicates success, 4xx are temporary failures that can be retried, while 5xx errors signal permanent failures needing troubleshooting. 
  15. What are key differences between push email protocols like SMTP versus pull protocols like POP3/IMAP?
    Push automatically notifies and transfers messages fast without users explicitly logging in and downloading unlike pull. Push offers more real-time delivery while pull allows scheduled batch access. 
  16. What are the most common record types used for routing SMTP mails?
    The most common DNS records enabling SMTP routing are MX records listing designated mail servers for domain, A records resolving hostnames to IPs, PTR records resolving IPs back to hostnames. 
  17. What are some alternatives to standard ASCII encoding used by legacy SMTP when sending special chars or non-English texts?
    Some alternatives adding support for special charsets and encoding are SMTPUTF8 for UTF8 chars, Internationalized email headers encoded using MIME formats etc. 
  18. How does SMTP “relaying” work and what prevents open mail relays?
    Earlier any server would relay messages from anyone. Now based on anti-spam rules, only authenticated users associated with domain can relay. Restricting outsider connections prevents abuse. 
  19. What are key differences between push email protocols like SMTP versus pull protocols like POP3/IMAP?
    Push automatically notifies and transfers messages fast without users explicitly logging in and downloading unlike pull. Push offers more real-time delivery while pull allows scheduled batch access. 
  20. What are some modern SMTP security best practices?
    Some key best practices are mandating TLS encryption, stronger authentication mechanisms over plain-text passwords, access restrictions, monitoring for anomalies, patches/upgrades etc.

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