SMTP (Simple Mail Transfer Protocol)

SMTP (Simple Mail Transfer Protocol) is the standard protocol used to send email over the Internet.

Whenever you send an email, your email client communicates with an SMTP server, which then delivers the message to the recipient's mail server.

SMTP is used only for sending email. Receiving email is typically handled by IMAP or POP3.

When you send an email, the following steps occur:

1. Your email client connects to an SMTP server.
2. The SMTP server authenticates the sender (if required).
3. The server looks up the recipient domain's MX record.
4. The sending server connects to the recipient's SMTP server.
5. The recipient's server accepts and stores the message.

Your Email Client
        │
        ▼
SMTP Server
        │
        ▼
DNS MX Lookup
        │
        ▼
Recipient SMTP Server
        │
        ▼
Recipient Mailbox

SMTP is a text-based protocol.

Typical conversation:

HELO example.com

MAIL FROM:<alice@example.com>

RCPT TO:<bob@example.net>

DATA

(message contents)

QUIT

Modern SMTP servers usually use EHLO instead of HELO because it supports additional features.

Today, email clients should normally use 587 with STARTTLS or 465 with implicit TLS.

Most SMTP servers require authentication before allowing users to send email.

Typical credentials:

• Username (usually the email address)
• Password

Without authentication, the server may refuse to relay messages.

Modern SMTP uses encryption to protect credentials and message transmission.

Two common methods are:

• STARTTLS (usually on port 587)
• Implicit TLS (usually on port 465)

Encryption helps protect against eavesdropping while data travels across the network.

An SMTP server that forwards email to another server is performing mail relay.

Example:

alice@example.com

↓

mail.example.com

↓

mail.example.net

↓

bob@example.net

Open relays (servers that allow anyone to relay email) are considered a major security risk and are almost always blocked.

SMTP relies heavily on DNS.

Before delivering an email, the sending server performs an MX lookup to determine which server accepts email for the recipient's domain.

Example:

example.net

↓

MX Record

↓

mail.example.net

If no valid MX record exists, delivery may fail.

SMTP servers return numeric status codes.

Common examples:

Codes beginning with:

• 2xx → Success
• 4xx → Temporary failure
• 5xx → Permanent failure

These protocols have different purposes.

SMTP sends messages, while IMAP and POP3 retrieve them.

The Telenegar Mail Check service can help diagnose SMTP-related problems such as:

• Incorrect MX configuration
• SMTP connectivity
• TLS availability
• Reverse DNS issues
• SPF, DKIM and DMARC configuration
• Common delivery problems

These checks help improve email deliverability.

• SMTP receives email.
  • False. SMTP sends email.

• Port 25 should be used by email clients.
  • Generally false. Email clients should normally use port 587 or 465.

• SMTP encrypts email by default.
  • False. Encryption is provided by TLS.

• Use port 587 with STARTTLS whenever possible.
• Enable SMTP authentication.
• Configure SPF, DKIM and DMARC.
• Ensure your server has valid reverse DNS (PTR).
• Use a valid TLS certificate.

• MX Records
• IMAP
• POP3
• SPF
• DKIM
• DMARC
• DNS Explained
• Mail Check

email smtp mail tls dns mx authentication