In simple terms:

Every time a webshop sends an order confirmation, a website sends a password reset, or a business sends a marketing email, SMTP is part of the delivery process.

SMTP stands for Simple Mail Transfer Protocol.

It is a standard set of rules that allows email systems to communicate with each other when sending and transferring email.

SMTP has existed since the early days of the internet, but it remains the foundation of email delivery today.

Even when an application sends email through an email API, SMTP is normally involved later when the provider transfers the message to the recipient's mail server.

An SMTP server is a mail server that accepts, processes, queues and transfers outgoing email.

Think of it as a specialized delivery system for email.

Your website, application or email program hands the message to an SMTP server. The SMTP server then determines where the message needs to go and attempts to deliver it to the recipient's mail provider.

For example:

or:

The SMTP server handles the technical communication required to move the email between these systems.

When you click Send, several things happen in just a few seconds.

1. The application connects to an SMTP server

The sending system could be Outlook, WordPress, a webshop, CRM, accounting software, SaaS platform, IoT device or custom application.

It connects to an SMTP server using a hostname such as:

smtp.example.com

Applications typically connect using SMTP port 587, 465 or sometimes 2525.

2. The SMTP connection starts

The sending application introduces itself using an SMTP command such as:

EHLO

The SMTP server responds with the features it supports, which can include authentication and TLS encryption.

If authentication is required, the application proves that it is authorized to use the SMTP server.

3. SMTP identifies the sender

The sending system provides the envelope sender using:

MAIL FROM:<sender@example.com>

This address is part of the SMTP envelope.

The SMTP envelope contains the routing information mail servers need to deliver the message.

It is separate from the From address that a person normally sees when reading the email.

4. SMTP identifies the recipient

The recipient is specified using:

RCPT TO:<recipient@example.net>

For messages with multiple recipients, the SMTP conversation can contain multiple RCPT TO commands.

5. The message is transferred

The sending application issues the:

DATA

command.

It can then transfer the actual email, including its headers, subject, text, HTML and attachments.

Once the SMTP server accepts the message, it can place it in its outgoing mail queue.

6. DNS finds the receiving mail server

The SMTP server now needs to find out where email for the recipient's domain should be delivered.

If the recipient is:

anna@example.net

the sending server performs a DNS lookup for the MX records of example.net.

MX stands for Mail Exchange.

These DNS records identify the mail servers responsible for accepting email for the domain.

7. The SMTP server delivers the email

The sending SMTP server connects to the receiving mail server, normally over port 25.

Another SMTP conversation takes place between the two servers.

The receiving server may examine the:

• sending IP address
• sender domain
• SPF authentication
• DKIM signature
• DMARC alignment
• reverse DNS
• sender reputation
• message characteristics
• recipient address

The receiving server then decides whether to accept or reject the email.

If accepted, it normally responds with an SMTP success code beginning with:

250

The SMTP transaction is complete.

The receiving provider then determines where the accepted message should be placed.

That could be the inbox, a category such as Promotions, or the spam folder.

The complete journey can be summarized as:

That is the basic infrastructure behind internet email.

An SMTP relay is a service that accepts email from your application and delivers it to other mail servers on your behalf.

Instead of your webshop, website or application communicating directly with thousands of different mail providers, it sends everything through an SMTP relay.

For example:

The SMTP provider handles the internet-facing delivery infrastructure.

SMTP relays are commonly used for:

• transactional email
• password resets
• login codes
• invoices and receipts
• webshop notifications
• SaaS applications
• CRM systems
• monitoring systems
• IoT devices
• newsletters
• marketing email

SMTP, IMAP and POP3 perform different jobs.

SMTP sends email.

IMAP gives email clients access to messages stored on a mail server and synchronizes them across devices.

POP3 downloads received messages from a mail server.

A simple way to remember the difference is:

SMTP therefore delivers a message to the recipient's mail system. IMAP or POP3 may later be used when the recipient accesses that mailbox.

Different SMTP ports are used for different parts of email delivery.

SMTP port 25

Port 25 is primarily used for SMTP communication between mail servers.

For example:

or:

Port 25 is frequently restricted by internet providers and hosting companies for end-user connections because of its historical use by spam and compromised systems.

Applications should normally use a submission port instead.

SMTP port 587

Port 587 is the standard SMTP message submission port and is usually the best default for applications.

It commonly uses STARTTLS, which upgrades the connection to TLS encryption.

SMTP port 465

Port 465 is used for SMTP submission with implicit TLS.

The TLS-encrypted connection is established immediately.

Port 465 is a valid modern SMTP submission option when supported by your provider.

SMTP port 2525

Port 2525 is a commonly supported alternative SMTP port.

It is useful when standard SMTP submission ports are restricted by a network or hosting provider.

It is not an official replacement for port 587, but many SMTP providers support it.

SMTP by itself was created long before today's internet security requirements.

Modern SMTP therefore uses additional security technologies.

TLS

TLS encrypts the connection between SMTP systems.

It protects SMTP credentials and message content from being transmitted in plain text across the connection.

But encryption is only one part of modern email security.

Three other technologies are especially important:

SPF

SPF (Sender Policy Framework) identifies which servers or IP addresses are authorized to send email for a domain.

DKIM

DKIM (DomainKeys Identified Mail) adds a cryptographic signature to an email.

The receiving mail server can verify the signature using a public key published in DNS.

DMARC

DMARC (Domain-based Message Authentication, Reporting and Conformance) builds on SPF and DKIM and checks their alignment with the domain visible in the From address.

DMARC also allows domain owners to publish a policy for messages that fail authentication.

For professional email sending, SMTP, TLS, SPF, DKIM and DMARC work together.

No. SMTP delivery and inbox placement are not the same thing.

This distinction is extremely important.

An SMTP server can successfully deliver an email to Gmail, Microsoft or Yahoo and receive:

250 OK

That means the receiving mail system accepted the message.

It does not necessarily mean that the email was placed in the inbox.

The receiving provider can still place the message in spam or another folder.

Inbox placement can be influenced by factors such as:

• IP reputation
• domain reputation
• SPF
• DKIM
• DMARC
• PTR / reverse DNS
• sending history
• sending volume
• spam complaints
• bounce rates
• mailing-list quality
• recipient engagement
• message content

This is why professional email delivery is about much more than simply having access to an SMTP server.

SMTP servers communicate using numeric response codes.

These codes tell the sending system whether a command or message was accepted.

The easiest way to understand them is by their first digit.

2xx — Success

The SMTP command or message was accepted.

For example:

250 OK

4xx — Temporary error

The server cannot accept the message right now.

Examples include temporary rate limits, server problems and reputation-related throttling.

A properly configured SMTP server will normally queue the message and try again later.

5xx — Permanent error

The receiving server has rejected the command or message.

Possible reasons include:

• invalid recipient
• blocked sender
• authentication problems
• policy violations
• spam filtering
• configuration errors

The text following the SMTP code often provides more information about why the message was rejected.

Applications can commonly submit email in two ways:

SMTP or an Email API.

SMTP uses the universal SMTP protocol and normally requires:

• SMTP hostname
• username
• password
• port
• encryption settings

An email API normally sends the message to the email provider using an HTTPS request.

APIs can provide tighter integration and additional functionality, but SMTP has one major advantage:

It works with an enormous range of existing software and devices.

You can often switch SMTP providers simply by changing the SMTP hostname, username and password without changing the application itself.

Running an SMTP server is possible.

Running one that consistently delivers legitimate business email is considerably more complicated.

A professional SMTP infrastructure needs to manage much more than the SMTP connection itself.

That includes:

• IP addresses
• IP reputation
• domain reputation
• sending queues
• retries
• bounce processing
• TLS
• SPF
• DKIM
• DMARC
• reverse DNS
• abuse prevention
• suppression
• monitoring
• changing requirements from receiving providers

For businesses, the real objective isn't simply:

"Can we send an email?"

It is:

"Will our important email reach the recipient reliably?"

A dedicated SMTP IP is a sending IP address used exclusively by one customer or organization.

With shared SMTP infrastructure, multiple customers send email through the same IP addresses.

With a dedicated IP, the sending organization has greater control over its own sending reputation.

A dedicated IP can be particularly useful for businesses sending regular volumes of transactional or marketing email where reputation, monitoring and predictable delivery are important.

However, a dedicated IP also needs to be managed correctly. Sending patterns, authentication, list quality and complaint rates all influence its reputation.

SMTP is the technology that moves email across the internet.

The basic journey is:

But modern email delivery involves more than SMTP.

For reliable business email, SMTP works together with TLS, DNS, SPF, DKIM, DMARC, reverse DNS and sender reputation.

That is the difference between simply sending email and operating a professional email delivery infrastructure.