What is the SMTP server used for?

The SMTP server is used for handling outgoing email transmission. It is responsible for routing and delivering emails via the Simple Mail Transfer Protocol (SMTP) from clients to recipient addresses through relaying between mail servers over the Internet.

What is the SMTP server used for?

Overview of SMTP Server

An SMTP server is an application that enables sending emails using the SMTP protocol:

  • Also known as mail transfer agent (MTA) or mail relay server.
  • Listens for requests on default SMTP port 25 to accept outgoing mails.
  • Verifies sender identity and authenticates if required.
  • Examines recipient address to determine destination mail server.
  • Establishes connection with destination server and relays message over.
  • Can route mail through multiple hops between servers across the Internet based on DNS.

So in essence, SMTP servers transport email messages between servers using standard SMTP protocol and routing.

Key Functions of an SMTP Server

The core functions of an SMTP server include:

  • Accepts outgoing email submission requests using SMTP.
  • Resolves domain names to find appropriate destination mail servers.
  • Adds email message content with headers to envelope for transmission.
  • Establishes connection with target mail servers and handshakes SMTP banner.
  • Relays messages across multiple hops till delivery as needed based on routing.
  • Sends back status codes and errors if issues in message acceptance or relaying.
  • Enforces administrator policies like permissions, restrictions.
  • Logs transactions and details like IPs, timestamps.

So reliable routing and delivery of emails between servers is made possible by SMTP servers.

Working of an SMTP Server

The typical flow in the working of an SMTP server is:

  1. Establishes listening TCP socket on port 25 to accept client SMTP connections.
  2. Accepts connection from sending Mail User Agent (MUA).
  3. Client initiates session by sending EHLO and HELO commands identifying itself.
  4. Server sends welcome message and states supported extensions.
  5. Client provides sender and recipient detail using SMTP envelope.
  6. Client initiates sending of message content with DATA command.
  7. Server accepts message content and buffers/stores temporarily if no errors.
  8. Server resolves domain name of recipient address to find destination SMTP server.
  9. Tries to relay message to destination server over a new connection.
  10. Receives status code on success or failure of relaying from remote server.
  11. Informs client accordingly about message transmission result.

So in summary, SMTP servers enable reliable hop-by-hop routing and delivery of emails between multiple mail servers over the Internet.

Common SMTP Server Software

Some commonly used SMTP server software implementations are:

  • Sendmail – Open source SMTP application used widely on Linux and Unix systems. Powerful and highly customizable but complex to administer.
  • Postfix – Another popular open source alternative to Sendmail. Comparatively simpler configuration while still being fast and secure.
  • Exchange Server – Windows Server role from Microsoft providing SMTP along with other collaborative services.
  • Gmail – Google’s email service uses its own proprietary SMTP servers for sending mails between data centers.
  • MailEnable – Reliable windows SMTP server also featuring webmail, security and integration capabilities.
  • hMailServer – Free lightweight SMTP application for Microsoft environments.

Organizations can choose suitable SMTP server software aligning with internal skill sets and operational capabilities.

Key Differences – SMTP Server vs Email Server

Though they work together, SMTP servers and email servers have distinct roles:

SMTP Server Email Server
Sends email between servers via SMTP protocol. Offers email access services to end users/clients.
Routes message to destination server based on domain name. Provides access to mailboxes for composing or reading messages.
Does not hold messages beyond transit. Retains messages in user mailboxes unless deleted.
Has no concept of users or mailboxes. Manages storage quotas, user access credentials.
Does not deal with authentication. Requires user verification for security.
Delivers email reliability. Provides email productivity features to users.
Traffic is server-to-server. Client-facing functions like IMAP/POP3.
Core protocol used is SMTP. Supports protocols like IMAP, POP3, HTTP etc.

So SMTP servers transport mail while email servers manage end user accounts, access and storage.

Importance of Properly Configured SMTP Servers

Well configured SMTP servers are vital for effective email delivery:

  • Reliability – Robust SMTP servers ensure messages reliably arrive without losses in transit.
  • Speed – Optimized routing, caching and parallelization improves delivery speeds.
  • Reputation – Servers with good sending reputation ensure emails reach inboxes without blocking.
  • Security – Proper authentication, TLS encryption, DKIM etc. harden security and prevent attacks.
  • Scalability – Ability to handle increased loads seamlessly during traffic surges using clustering.
  • Monitoring – Well monitored systems prevent outages, detect threats early.
  • Redundancy – Prevents single point of failure through failover methods.

So properly administering SMTP infrastructure including access policies, security principles, redundancy etc. is key for performance and delivering user value.

Key Considerations when Selecting an SMTP Server

Important aspects to evaluate when choosing an SMTP service or software include:

  • Protocols and extensions support – TLS, SPF, DKIM etc.
  • Ease of administration and integration into existing environment.
  • Scalability and throughput capacity to handle sent mails.
  • Delivery performance and reliability globally.
  • Reputation history and delisting likelihood for emails landing in spam.
  • Authentication mechanisms available for access control.
  • Customization flexibility for unique needs.
  • Cost of software licensing, operations and maintenance.
  • Availability of technical support and managed services if required.

So requirements around capacity, security, integration complexity and costs drive SMTP solution decisions.

Key Takeaways on SMTP Servers

  • SMTP servers act as mail transfer agents that enable sending emails between mail servers over the Internet.
  • They use SMTP protocol to accept outgoing messages and relay them to destination mail servers based on domain names.
  • Fast and reliable message routing, security, reputation, scalability etc. depend on robust SMTP server configuration.
  • Separate email servers provide user-facing access services while SMTP servers handle server-to-server mail transmission in the backend.
  • Well optimized SMTP server infrastructure is vital for mailbox delivery, security and performance.


The SMTP server plays a pivotal role in email transmission by accepting outgoing mails and reliably delivering them to recipients’ mail servers using standardized SMTP-based routing over TCP/IP networks. It forms the gateway for clients to submit messages to be relayed ahead. Careful selection, configuration and monitoring of SMTP servers ensures fast, scalable and secure mail transport. While hidden from end users, SMTP servers enable ubiquitous worldwide email connectivity by allowing seamless interoperability between diverse mail servers over the resilient SMTP protocol that successfully withstood the test of time.


  1. Can an email server also work as SMTP server?
    Yes, email servers like Microsoft Exchange play the dual role – accepting incoming messages from SMTP servers and providing outgoing SMTP services.
  2. What are disadvantages of using ISP’s SMTP server?
    Drawbacks include lower message limits, monitoring, delays, instability, IP blocks and possibility of flagging as spam since shared by their entire user base.
  3. Can I use Gmail’s SMTP server?
    No, Gmail restricts its SMTP servers usage only for Gmail accounts. Users need to use authenticated submission over SMTP or configure a dedicated SMTP service.
  4. What are benefits of cloud-based SMTP servers?
    Benefits include scalability, reliability, reputation, simplified security patching and compliance overheads, reduced hardware maintenance etc. when using managed services.
  5. Is Windows Server or Linux better for SMTP?
    Both have comparable performance. Linux is easier to customize being open source. Windows Server offers closer integration into Microsoft enterprise ecosystem.
  6. What are disadvantages of self-hosted SMTP servers?
    Self-hosting increases hardware, setup complexity, ongoing security and patch management overheads. Lack of reputation can lead to deliverability issues.
  7. How does an SMTP server identify the destination for an email?
    SMTP servers resolve the domain name of recipient addresses specified in SMTP envelope into destination IP addresses through DNS lookups to identify destination mail servers.
  8. Can I use SMTP server without domain and website?
    Yes, you can use third-party SMTP services for transactional or marketing emails without needing to have a domain or website, similar to mail merge.
  9. How are corporate SMTP servers made secure?
    Corporate SMTP servers are hardened using transport rules, TLS encryption, DKIM signatures, SPF policies, Access Control Lists restricting relays, IP allow-lists etc.
  10. What ports does SMTP use?
    Primary SMTP uses port 25. Port 465 used for implicit TLS authentication. Port 587 is for SMTP mail submission from clients. Port 2525 also sometimes used for submissions.
  11. Do web servers use SMTP?
    Typically no. Web servers deal with HTTP(s) traffic. They may pass emails to be sent to an SMTP sever in the backend through code like PHP mail() function or via APIs.
  12. Can I use SMTP server without DNS records?
    No, SMTP functionality relies on DNS – A records identify server IP and MX records designate mail server hostname for routing to domains.
  13. Is SMTP used for incoming or outgoing emails?
    SMTP is used for outgoing, server-to-server mail delivery. Incoming mail pickup uses protocols like POP3 and IMAP instead that retrieve messages into user inboxes.
  14. Can my IP get blacklisted if SMTP is misconfigured?
    Yes, issues like open relays or suspicious traffic from misconfigured SMTP server can lead to blacklist entries blocking mail receiving by domains and hurting deliverability.
  15. How are large SMTP environments scaled?
    Large SMTP deployments use multi-server setups behind load balancers. Clustering, anycast etc. balance load. Solutions like Exchange also support multiple roles.
  16. Does SMTP work over HTTP?
    No. SMTP strictly operates directly over TCP. HTTP works at the application layer and deals with web content and traffic vs SMTP focusing on email delivery.
  17. How are SMTP log files useful?
    Log files provide useful diagnostics and audit trails containing timestamps, IP addresses, transmission details etc. to identity issues and monitor server activity.
  18. What is the difference between SMTPS and SMTP?
    SMTPS initiates TLS encryption implicitly at connection start while SMTP upgrades plain text connections to TLS using the STARTTLS command.
  19. What are disadvantages of cloud-hosted SMTP solutions?
    Possible disadvantages include monthly costs, vendor dependencies, lack of full customization flexibility vs self-hosted and risks of termination due to violations in usage terms.
  20. Do email clients use SMTP?
    Yes, email client apps and software use SMTP to send outgoing mails either directly or via submission port 587 to the organization’s SMTP server which relays messages onward.

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