Simple Mail Transfer Protocol (SMTP)

SMTP is an application-layer protocol used to send and transfer email between servers. It relies on a TCP connection to port 25 to reliably deliver messages from a client to an email server.

• Uses TCP for reliable message transmission
• Servers listen on port 25 for incoming email
• Client establishes a TCP connection before sending mail
• Core protocol for email delivery between mail servers

Types of SMTP Protocol

The SMTP model supports two types of email delivery methods: end-to-end and store-and-forward.

• End-to-end delivery is used between organizations. In this method, the email is sent directly from the sender's SMTP client to the recipient's SMTP server without passing through intermediate servers.
• Store-and-forward is used within organizations that have TCP/IP and SMTP-based networks. In this method, the email may pass through several intermediate servers (Message Transfer Agents, or MTAs) before reaching the recipient.

• SMTP.com: Platform providing transaction email, relay, and delivery services at affordable prices.
• Industry Trust: Regarded as the most trusted sender by ISPs with decades of experience.
• Customer Base: Trusted by over 100,000 customers over the years.
• User-Friendly: Extremely intuitive and easy to set up for seamless integration.
• Business Integration: Seamlessly integrates into existing business systems.
• Easy Migration: Effortless migration from other email providers.

Features

The following are the key features of SMTP.com:

• Dedicated IP: Provides a unique sending IP to improve deliverability and maintain a strong sender reputation.
• Email API: Enables fast and seamless integration with detailed documentation, allowing businesses to connect in just minutes.
• 24/7 Customer Support: Offers round-the-clock human support across all plans, including website assistance and live chat for quick, reliable solutions.
• High-Volume Sending Solutions: Ideal for enterprises sending over 250 million emails monthly, with customized quotes and tailored infrastructure.
• Reputation Defender: An add-on service that automatically scans, cleans, and monitors email lists to maintain healthy sender reputation—no integration required.

Model of SMTP System 

• User at a Terminal (Sender): The person composing the email using a mail application (e.g., Outlook, Thunderbird, webmail).
• User Agent (Sender): The email client software that allows the sender to create, edit, and send emails.
  It hands the outgoing email to the system’s mail queue.
• Queue of Mail to Be Sent: A temporary storage area where outgoing emails wait before being processed by the mail server (Message Transfer Agent).
• Message Transfer Agent (Client Side): The software responsible for sending emails across the network. It retrieves messages from the outgoing queue and communicates with the receiving server.
• TCP Connection (Port 25): The client’s MTA opens a TCP connection to the server’s MTA on port 25. SMTP commands, responses, and mail data are exchanged over this connection.
• Message Transfer Agent (Server Side): The receiving email server accepts the incoming SMTP connection. It processes the incoming email and stores it for the user.
• User Mailboxes: The server stores the received messages in individual user mailboxes.
• User Agent (Receiver): The email application used by the recipient to access the mailbox. It retrieves emails using protocols like POP3 or IMAP.
• User at a Terminal (Receiver): The person reading the received email on their device.

Components of SMTP

• Mail User Agent (MUA): It is a computer application that helps you in sending and retrieving mail. It is responsible for creating email messages for transfer to the mail transfer agent(MTA).
• Mail Submission Agent (MSA): It is a computer program that receives mail from a Mail User Agent(MUA) and interacts with the Mail Transfer Agent(MTA) for the transfer of the mail.
• Mail Transfer Agent (MTA): It is software that has the work to transfer mail from one system to another with the help of SMTP.
• Mail Delivery Agent (MDA): A mail Delivery agent or Local Delivery Agent is basically a system that helps in the delivery of mail to the local system. 

How does SMTP Work?

1. Sending Email:

• When a user wants to send an email, they use a User Agent (UA), like Outlook or Gmail.
• The email is handed over to the MTA, which is responsible for transferring the email to the recipient’s mail server.

2. SMTP Client and Server:

• Sender-SMTP (Client): The email sender’s MTA initiates the connection to the recipient’s MTA (Receiver-SMTP).
• Receiver-SMTP (Server): The receiving MTA listens for incoming connections and receives the email from the sender-SMTP.
• This communication happens over TCP port 25.

3. Relays and Gateways:

• Relays: In some cases, the email may pass through several intermediate MTAs before reaching the destination server. These MTAs act as relays.
• Gateways: If the sending and receiving systems use different email protocols (e.g., SMTP and non-SMTP), an email gateway can convert the email to the appropriate format for delivery.

4. Email Delivery:

• The sender’s MTA sends the email to the receiver’s MTA, either directly or through relays.
• The MTA uses the SMTP protocol to transfer the message. Once it’s delivered to the destination MTA, the email is placed in the recipient’s mailbox.
• The recipient’s User Agent (UA) can then download the email.

SMTP Envelope

Purpose

• The SMTP envelope contains information that guides email delivery between servers.
• It is distinct from the email headers and body and is not visible to the email recipient.

Contents of the SMTP Envelope

• Sender Address: Specifies where the email originates.
• Recipient Addresses: Indicates where the email should be delivered.
• Routing Information: Helps servers determine the path for email delivery.

Comparison to Regular Mail

• Think of the SMTP envelope as the address on a physical envelope for regular mail.
• Just like an envelope guides postal delivery, the SMTP envelope directs email servers on where to send the email.

SMTP Commands

SMTP Ports

These ports play a important role in defining how email clients and servers communicate securely and efficiently across the internet using the SMTP protocol.

• Port 587: This is the most commonly used port for secure SMTP submission using TLS (Transport Layer Security). It is recommended for client-to-server communication, as it ensures the security of the email transmission.
• Port 465: Previously used for secure SMTP (SMTPS), this port is no longer considered an official standard and is generally not recommended anymore. Many email providers have moved away from port 465 in favor of port 587.
• Port 25: This port is traditionally used for SMTP relay between mail servers, not for email submission from clients. It is often blocked by ISPs for outgoing mail due to its frequent use for spam and malicious activities.
• Port 2525: Although not an official SMTP port, it is sometimes used as an alternative for SMTP submission, especially in cases where port 25 is blocked or restricted. Many email providers support this port as an alternative for secure communication.

Difference Between SMTP and Extended SMTP

Advantages of SMTP

• If necessary, the users can have a dedicated server.
• It allows for bulk mailing.
• Low cost and wide coverage area.
• Offer choices for email tracking.
• Reliable and prompt email delivery.

Disadvantages of SMTP

• SMTP's common port can be blocked by several firewalls.
• SMTP security is a bigger problem.
• Its simplicity restricts how useful it can be.
• Just 7-bit ASCII characters can be used.
• If a message is longer than a certain length, SMTP servers may reject the entire message.
• Delivering your message will typically involve additional back-and-forth processing between servers, which will delay sending and raise the likelihood that it won't be sent.

SMTP vs POP vs IMAP