What is SMTP connection?

The Simple Mail Transfer Protocol (SMTP) is a communication protocol used for sending email messages between servers over the internet.

What is SMTP connection?

How SMTP Works

When you send an email from your email client or app, here are the basic steps involved:

  • Your email client connects to your organization’s SMTP server using port 25 or 587 and establishes a connection. This allows the email client to communicate with the SMTP server.
  • Your email client provides information like the sender’s email address, recipient’s email address, email content, etc.
  • The SMTP server verifies if the provided sender email ID is valid and authorized to send emails.
  • If valid, the SMTP server sends the email content and relevant information to the recipient’s SMTP server using the domain name in the recipient’s email address.
  • The recipient SMTP server verifies if the provided email ID is valid and exists in its database.
  • If valid, it deposits the email into the recipient’s inbox where the recipient can access it using their email client.

So in summary, SMTP is the protocol used by email servers to route your emails over the internet to the correct destination SMTP server which delivers it to the recipient’s email inbox.

Key Components of SMTP

There are some key components and processes in the SMTP system:

  • SMTP Banner: This is the initial banner text that identifies the SMTP server to connecting clients. It specifies the hostname, server name, software version, etc.
  • SMTP Commands: These are special text commands like EHLO, MAIL FROM, RCPT TO, DATA, QUIT etc. used by email clients to communicate with SMTP servers.
  • SMTP Replies: These are 3-digit reply codes like 250, 550, etc. that SMTP servers send in response to client commands indicating whether a command was successfully processed or not.
  • MX Records: These are special DNS records used by SMTP servers to route emails to the appropriate destination SMTP server.
  • SMTP Handshake: This is the initial connection dialogue between an email client and SMTP server used to establish communication.
  • SMTP Envelope: This refers to the transit information for an email message used for routing including To, From, Return-Path etc.
  • SMTP Validation: This involves validation checks performed by SMTP servers for aspects like sender address, recipient address, domain availability etc.

Understanding these key technical elements is important for gaining a deeper insight into the SMTP infrastructure.

Main Advantages of SMTP

Some of the main advantages of using SMTP as the email transport protocol include:

  • Simplicity – SMTP utilizes simple ASCII text commands which makes troubleshooting and analysis easy.
  • Ubiquity – SMTP servers are universally available across the internet making email delivery between different domains possible.
  • Extensibility – SMTP supports extensions through EHLO interface allowing additional features to be easily introduced.
  • Interoperability – Open SMTP standards allow different vendor implementations to easily intercommunicate.
  • Security – Communication channels can be easily encrypted using SSL/TLS providing good email security.
  • Reliability – Store and forward model, sent items persistence etc. make email delivery reliable.

These capabilities have made SMTP the ubiquitous standard for message transport across the internet for decades.

Common SMTP-Related Terminology

Here are some common technical terms associated with SMTP services:

  • SMTP Server: Software/hardware responsible for handling email routing and delivery using SMTP protocol.
  • SMTP Port: Commonly used ports – Port 25 (non-encrypted) and Port 587 (submission/encrypted)
  • SMTP Client: Email client software in end-user devices communicating with SMTP servers.
  • SMTP Relay: SMTP component that forwards email to next server.
  • SMTP Gateway: Software that converts messages from another format into SMTP.
  • ESMTP: Extended SMTP adds additional functionality while being compatible with SMTP.
  • SMTP Authentication: Uses authorization protocols like SCRAM or OAUTH 2.0 to securely authenticate clients.
  • SMTP TLS: Adds Transport Layer Security (TLS) for secure encrypted connections.

Familiarity with these key terms will provide better understanding of how SMTP technology functions.

Key Differences Between POP3 and IMAP

POP3 and IMAP are two protocols used by email clients to retrieve messages from mail servers:

  • POP3 – Post Office Protocol v3 is simpler and downloads messages from server onto the client machines deleting them from the server.
  • IMAP – Internet Message Access Protocol keeps all messages on the server allowing access from multiple client devices.

Here is a comparison between the two protocols:

Parameter POP3 IMAP
Access Temporary Continuous
Message Sync Download on client Access from server
Multiple Device Access No Yes
Storage Usage Minimal More
Deletion Sync Manual Automatic

So while POP3 offers a simpler approach, IMAP provides more flexibility and features for modern day access from diverse sources.

Steps Involved in Sending and Receiving Emails

The typical sequence of events involved in sending emails from clients and accessing those emails are:

Sending Emails

  1. Email client connects to outbound SMTP server using SMTP handshake
  2. Client sends email content, sender and recipient details as SMTP commands
  3. SMTP server validates the sender and recipient addresses
  4. Server sends email info and content to destination using SMTP envelope and routes
  5. Destination SMTP server deposits it into recipient’s mailbox

Receiving Emails

  1. Email client connects to incoming mail server using POP3 or IMAP
  2. Mail server authenticates the client based on credentials
  3. Client issues commands to access inbox listings and email content
  4. Messages are downloaded locally or accessed from server based on POP3 or IMAP
  5. Client can then access, read, delete or archive messages as per convenience

Understanding this sequence of events helps troubleshoot issues for non-delivery or access of emails.

Key Best Practices for SMTP Servers

Some vital best practices for configuring and maintaining SMTP servers include:

  • Always use the most updated stable version of the mail transfer agent software like Postfix, Sendmail, etc.
  • Enable domainkeys and DKIM signatures for outgoing messages to improve deliverability.
  • Enforce TLS encryption for secure connections using digital certificates.
  • Configure proper DNS records like MX, A, PTR for your domain.
  • Monitor server load and bandwidth usage to judge capacity needs.
  • Log all activity meticulously for effective troubleshooting and analysis.
  • Implement strong authentication mechanisms like SPF, DKIM etc.
  • Leverage blacklists to block spammers.
  • Set rate limiting policies to avoid being flagged as source of spam.
  • Ensure adequate security measures against attacks and unauthorized access.

Following these best practices will result in higher performance and deliverability for your SMTP servers.

Key Takeaway

  • SMTP or Simple Mail Transfer Protocol facilitates transmission of email messages between servers over the Internet
  • Email clients use SMTP for sending out emails while protocols like POP3 and IMAP allow accessing received emails
  • It relies on direct server-to-server communication for routing using DNS records and SMTP envelope information
  • Understanding basics of how protocols like SMTP, POP3 and IMAP function helps diagnose email issues
  • Implementing key best practices is vital for ensuring effective management of your SMTP infrastructure

So SMTP forms the fundamental building block that has allowed email to become such an indispensable communication mechanism globally.


SMTP plays a foundational role in enabling email communication by facilitating sending of messages between participating servers. Learning SMTP helps one gain a solid grasp of email routing basics. As securing inboxes becomes critical over time, following best practices around encryption, deliverability and security when operating SMTP infrastructure has become imperative than ever. With strong growth prospects for email communication well into the future, the relevance of running robust and efficient SMTP servers is bound to carry on.

Frequently Asked Questions 

Q1: What is SMTP?
A1: SMTP or Simple Mail Transfer Protocol is the protocol used for communication between email servers to send mail over the internet by defining the envelope containing sender & recipient addresses, message content etc.

Q2: Which port does SMTP use?
A2: SMTP commonly uses Port 25 for non-encrypted connections. Port 587 is also used for encrypted SMTP connections.

Q3: What is the difference between SMTP and POP3?
A3: SMTP is used for sending emails while POP3 (Post Office Protocol 3) is typically used by email clients to retrieve emails from mail servers and download it locally.

Q4: What is SMTPS?
A4: SMTPS refers to Simple Mail Transfer Protocol Secure which runs SMTP over an encrypted SSL/TLS connection to secure email traffic during transit.

Q5: What is the SMTP (envelope) return path?
A5: The return path is the email address specified in the SMTP envelope using the SMTP MAIL FROM command. It indicates where to send bounced messages in case of delivery issues.

Q6: Can we send emails without an SMTP server?
A6: No, you cannot send emails over the internet without using one or more SMTP servers for routing messages to the recipient server.

Q7: How does SMTP know where to send email?
A7: SMTP uses the domain part of the email address to determine the recipient mail server via DNS MX records which specifies the SMTP mail servers for a domain.

Q8: What is an SMTP gateway?
A8: An SMTP gateway acts as the interface to transform incoming messages from another protocol/format into the SMTP format before routing towards destination over the internet.

Q9: What is SMTP authentication?
A9: SMTP authentication allows clients and servers to authenticate each other securely before sending messages. It uses mechanisms like OAuth, CRAM-MD5 etc.

Q10: How to check if SMTP server is working?
A10: You can use the Telnet client to connect via ports 25 or 587 and issue SMTP manual commands and observe server response codes to check if an SMTP server is working.

Q11: What is an SMTP relay?
A11: An SMTP relay is an SMTP server that receives messages from another server or email client and transmits it towards its destination instead of being the final delivery SMTP server.

Q12: What error does SMTP “Mailbox unavailable” mean?
A12: The “mailbox unavailable” SMTP error typically occurs when the destination email mailbox specified cannot be located or does not exist in the SMTP mail server database.

Q13: Can I send 100,000 emails via SMTP?
A13: You can but ISPs often set limits or may flag it as spam. It is better to use an email service provider with dedicated infrastructure for sending bulk commercial emails at scale.

Q14: What is SMTP domain?
A14: An SMTP domain refers to the domain name configured on an SMTP server having DNS records allowing Internet hosts to route SMTP traffic for users of that domain name via the designated mail servers.

Q15: What is an SMTP header?
A15: An SMTP header contains metadata like To, From, Subject etc. about the email message that is transmitted at the start of SMTP envelope which encompasses the header and message body.

Q16: Can we use Gmail SMTP server?
A16: Yes, Gmail provides SMTP servers for their users – smtp.gmail.com on port 587 with SSL/TLS for secure delivery of outbound messages after authentication.

Q17: Is SMTP insecure?
A17: The original SMTP does have some vulnerabilities but extensions like SMTPS force all connections to use SSL/TLS while DKIM & DMARC provide signature validation making emails more secure.

Q18: What is the default SMTP port for Outlook?
A18: For Outlook, the default outgoing SMTP port is 25 and 587 while incoming server uses POP3 port 110 and IMAP port 143. Encrypted connections use extra security with TLS.

Q19: What is SMTP user authentication?
A19: SMTP user authentication enables verifying through mechanisms like OAuth 2.0 that the user logging in to SMTP is valid, authorized and authenticated to prevent identity spoofing when sending emails.

Q20: Can I use Gmail SMTP server for business?
A20: You cannot use Gmail’s SMTP server directly for business bulk emails. But their GSuite business offerings do come with dedicated SMTP servers optimized for sending high volumes of business emails.

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