What is the difference between SMTP and SMTP server?

The Simple Mail Transfer Protocol (SMTP) is a communication protocol that allows the transmission of electronic mail messages between servers and clients. An SMTP server is a software application running on a computer that sends, receives, and routes SMTP email messages.

What is the difference between SMTP and SMTP server?

Key Differences Between SMTP and SMTP Server

  • SMTP is a standard protocol used for sending emails across the Internet. It defines the message format and transmission steps involved in email delivery.
  • An SMTP server is a software application that handles sending, receiving, and routing email messages by using the SMTP protocol. Popular SMTP servers include Exchange, Sendmail, Postfix, etc.

Here is a quick overview of some of the main differences:

  • Nature: SMTP is a protocol. SMTP server is an application software.
  • Function: SMTP defines how data is formatted and transmitted. SMTP server sends and receives SMTP formatted messages.
  • Use: SMTP is a communication mechanism used by email. SMTP server enables email services by handling SMTP messages.
  • Access: SMTP is a universal open standard protocol. SMTP server is proprietary server software accessed by authorized users and applications.

How SMTP Protocol Works

The SMTP process involves the following basic steps:

  • The sending mail client establishes a connection with the originating SMTP server on ports 25, 465 (over SSL) or 587 (for TLS encryption).
  • The sending SMTP server determines if the destination email server is accessible, resolving any DNS entries if needed.
  • The sending SMTP server establishes a connection with the receiving SMTP server and negotiates communication options like authentication mechanisms and encryption protocols to use.
  • The sender’s SMTP server sends the formatted email message to the receiving SMTP server.
  • The receiving SMTP server accepts and processes the incoming message for the recipient’s mail server.
  • The receiving SMTP server sends a delivery status notification to the sending SMTP server of whether the message transmission succeeded or failed.

So in summary, SMTP is the fundamental standard protocol used by all email servers to transfer email messages from the sender’s to the recipient’s email servers across the Internet.

Role and Functioning of SMTP Email Servers

SMTP email servers perform several key functions in handling SMTP messages:

Sending Emails

  • Accepts outgoing email messages from email clients like Outlook or Thunderbird addressed to external email domains.
  • Resolves domain name DNS records of receiving servers.
  • Establishes SMTP connections with receiving email server domains.
  • Formats messages according to SMTP standards and transmits them to receiving servers.

Receiving Emails

  • Listens and accepts incoming SMTP connections and messages from external sending domains.
  • Processes and filters incoming emails for spam, malware and routing to internal mailboxes.
  • Sends SMTP delivery status notification back to sending server.

Routing and Delivery

  • Routes incoming emails to correct internal mailboxes based on email address.
  • Maintains message queue of emails scheduled for delivery retries.
  • Attempts final delivery into user mailboxes like Exchange or email clients.

So essentially, an SMTP server manages all the SMTP mail transmission tasks of connecting to other SMTP servers, formatting mail messages according to SMTP protocol and orchestrating reliable end-to-end delivery of emails.

Major SMTP Email Servers

Some popular SMTP server platforms used today include:

  • MS Exchange Server: Microsoft’s enterprise email server integrated with Office 365 and Windows Server.
  • Sendmail: A legacy open-source SMTP system common on Linux/UNIX machines.
  • Postfix: Another popular open-source SMTP server for Linux/UNIX systems.
  • Gmail: Google’s web-based email platform also functioning as an SMTP gateway service.
  • Amazon SES: AWS cloud-based SMTP service for high volume email deliverability.
  • Mailgun & SendGrid: Hosted SMTP services focused on email deliverability and analytics.

These SMTP servers allow setting up business email services, providing advanced security, automation and analytics around managing high volume SMTP mail flow.

Key Differences

Standard protocol for email transmission Software application implementing SMTP protocol
Specifies email message formatting, routing, delivery Sends, receives, routes and delivers SMTP formatted email
Typically listens on ports 25, 465 or 587 Listens on port 25 to accept SMTP connections
Language-independent communication mechanism Usually written in languages like Java, C++, C# etc
Defines sequence of communication between sender & receiver’s mail servers Handles all aspects of sending & receiving emails

So in summary:

  • SMTP is the fundamental mail transfer protocol enabling email across the Internet
  • SMTP server is the critical software that handles sending & receiving SMTP message traffic between different email servers to reliably deliver messages to user inboxes.

Key Takeaways

  • SMTP is a universal communication protocol that formats email messages and manages transmission between email servers across the Internet.
  • An SMTP server is a specialized software application that sends, receives, routes and delivers SMTP-formatted email messages reliably between email domains.
  • Understanding the difference allows distinguishing the software that enables email services from the fundamental language that makes global email transmission possible.
  • SMTP servers implement SMTP protocol rules to communicate with external email servers and transmit emails reliably to and from email domains worldwide.


In essence, SMTP is the standardized protocol that defines common message formatting and delivery mechanisms that allows global interoperable email services across disparate networks and servers.

SMTP servers provide the critical software infrastructure that handles reliable sending, receiving and routing of SMTP email traffic between different domains and recipients worldwide. Most email messages transmitted across the Internet utilize SMTP protocol rules and SMTP servers to facilitate seamless global communication.

Frequently Asked Questions

  1. What is SMTP protocol used for?
    SMTP protocol is universally used to format email messages and reliably transmit them between sender & receiver mail servers across the internet to facilitate global email services.

  2. What port does SMTP use?
    SMTP typically communicates on port 25 as well as ports 465 (SMTP with SSL) and 587 (SMTP with TLS encryption).

  3. What is the difference between SMTP and IMAP?
    SMTP handles sending emails while IMAP deals with accessing received emails from mailboxes for end-user email clients.

  4. Is Gmail SMTP or IMAP?
    Gmail is both an SMTP server for sending emails as well as an IMAP server for allowing users to access their mailboxes via email clients.

  5. What is the most popular SMTP server?
    Some popular SMTP platforms are MS Exchange Server, Sendmail and Postfix on Linux/UNIX systems, and cloud-based services like Amazon SES, Mailgun and SendGrid.

  6. How does an SMTP server work?
    An SMTP server sends & receives SMTP formatted messages, routes emails to correct mailboxes and handles retries to ultimately deliver emails from senders to recipients’ email servers.

  7. Is SMTP insecure?
    Basic SMTP has security vulnerabilities but extensions like SMTPS (SMTP over SSL) and STARTTLS (upgrades to TLS encryption) significantly improve SMTP security.

  8. What is SMTP authentication?
    SMTP authentication allows the SMTP server to verify a user’s identity through login credentials before allowing email sending via protocols like CRAM-MD5 and AUTH LOGIN.

  9. What are SMTP headers?
    SMTP message headers contain metadata like sender, recipients, routing info, subject line etc that accompany the email body to provide context to the receiving SMTP server.

  10. Can I send emails without an SMTP server?
    To send emails across domains, an SMTP server is required to speak SMTP protocol to transmit messages to external recipient addresses. However, applications can send SMTP messages internally between their own users without an email server.

  11. What is the difference between SMPT and SMTPS
    SMTP uses plain text connections while SMTPS enhances security by encrypting the SMTP tunnel with SSL/TLS minimizing risk of data leaks during email transmission.

  12. Why do I need an SMTP server?
    The SMTP server handles the process of reliably coordinating delivery of email to and from external domains based on Internet standards – essential for global business email communications to function.

  13. What is SMTP authentication mechanism?
    It is a way for SMTP server to validate identity of sender via login credentials through protocols like CRAM-MD5, PLAIN or AUTH-LOGIN before allowing access to send emails.

  14. Can you explain SMTP message flow?
    A typical SMTP message flow involves – client connects to mail server on port 25/587, handles authentication, sender transmits SMTP formatted mail, server issues response codes on successful/failed delivery.

  15. Is Gmail SMTP or POP3?
    Gmail actually provides both SMTP server for sending emails as well as POP3 and IMAP protocols to enable checking Gmail mailboxes using email client software.

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