What are the two parts of SMTP?

The Simple Mail Transfer Protocol (SMTP) is a standard protocol used for sending email messages between servers over the Internet. SMTP has two main components:

What are the two parts of SMTP?

SMTP Client

The SMTP client is the software that constructs the email message, initiates the SMTP session, and sends the message content. Some examples of SMTP client software include:

    Email clients like Outlook, Thunderbird, Apple Mail

    Webmail interfaces like Gmail, Outlook, Yahoo! Mail

    Mobile apps that send email

    APIs that provide email sending capabilities within applications

The SMTP client handles creating the email content, formatting the message for transmission, initiating handshaking with the SMTP server, and transmitting the message over the network.

SMTP Server

The SMTP server is software that receives the email from the client and takes responsibility for routing the message towards its final destination. Common SMTP servers include:

    Gmail SMTP servers

    Outlook.com SMTP servers

    ISP email servers like Comcast, Verizon, AT&T

    Dedicated email servers like Microsoft Exchange

The SMTP server processes the incoming message, adds tracing information to the email header, determines the path to deliver the email based on DNS records, and relays the mail to the next SMTP server closer to its ultimate recipient or directly into the recipient’s mail server.

So in summary – the SMTP client composes and sends the email, while the SMTP server receives and routes the email across networks. Both the client and server are necessary for seamlessly transferring an email message between the sender and recipient.

How do SMTP clients format messages?

SMTP clients format email messages according to the specifications in RFC 5322. This standard defines the structure of Internet email messages, including components like:

    Header fields – To, From, Subject, Date

    Message body – The email content either as plain text or multimedia

    Boundaries – To delimit different parts of the message body

The client software constructs the message header with information like sender & recipient email addresses. It establishes the message body content and structure. And formats any attachments according to MIME encoding rules.

So in essence, the SMTP client packages up the email into a standardized format that any SMTP server will be able to process and transmit towards its destination.

What happens during SMTP transmission?

When an SMTP client sends an email, it establishes a SMTP session with the destination SMTP server. The process involves:

    Handshaking – The client authenticates itself with the SMTP server

    Mail transaction – The email content is transmitted

    Closure – The SMTP session is closed

In more detail:

    Session initiation starts with a handshake between client and server. This establishes an active SMTP connection.

    The client sends SMTP commands like MAIL, RCPT, DATA to transmit the actual message content.

    The server sends response codes like 220, 250 to indicate whether message transmission succeeded.

    Session closure happens with the QUIT command which terminates the SMTP session between client and server.

So essentially, the SMTP transmission process involves formatted message content being transmitted in anenvelope across an interactive SMTP session between client and server.

How do SMTP servers route messages?

SMTP servers route messages closer towards their destination by looking up DNS MX records. The MX records map a recipient’s domain to that domain’s mail server.

The routing process involves:

    Accept message from sending SMTP client

    Extract recipient email domain from RCPT command

    Lookup MX record for recipient domain

    Relay message to recipient mail server

If the recipient email address is not on the same domain as the SMTP server, it will repeat this process until the message reaches the recipient SMTP server. That server typically places the email in the correct mailbox where it can be accessed by the recipient’s email client.

So SMTP servers form a transient end-to-end mail delivery chain relying on DNS and hop-by-hop message relaying.

Key Takeaways

    SMTP has two components: client and server

    SMTP client composes and sends email

    SMTP server relays and routes email

    Clients format messages per RFC 5322 standard

    Transmission involves handshaking and message sending

    Servers route mails based on DNS MX records


In summary, SMTP operates using a client-server architecture to facilitate the sending and relaying of email across the Internet. All email transmission starts with an SMTP client creating and sending the formatted message content within an SMTP envelope. This is transmitted hop-by-hop between SMTP servers towards the final recipient’s mail server. Understanding this client-server interaction and transmission process is key to troubleshooting email delivery issues.

Frequently Asked Questions

  1. What is SMTP stand for?
    SMTP stands for Simple Mail Transfer Protocol. It is a standard for electronic mail transmission across Internet networks.

  2. Is SMTP client or server?
    The SMTP client is any software or application that generates and sends email. The SMTP server is software that receives email for routing towards final delivery.

  3. What port does SMTP use?
    SMTP commonly communicates over port 25 for insecure connections. Ports 465 (SMTP with SSL) and 587 (SMTP with TLS) are also used.

  4. Is Gmail SMTP or POP?
    Gmail is an SMTP server for sending emails. It supports POP and IMAP for retrieving emails from Gmail accounts.

  5. What tools can test SMTP servers?
    Some SMTP server testing tools include Telnet, swaks, MailTester, MailSlurper, and smtp-sink.

  6. Can you explain SMTP relays?
    An SMTP relay receives and forwards mail between unrelated parties. An open relay indiscriminately relays any mail without restriction.

  7. What is the difference between direct delivery vs relay?
    Direct delivery sends the email message straight to the recipient’s mail server. A relay instead forwards the mail to an interim mail server for further routing.

  8. What are common SMTP headers?
    Common SMTP headers include From, To, Cc, Subject, Message-ID, Date – specifying sender, recipients, topic and transmission details.

  9. What is SMTP authentication?
    SMTP authentication is used by mail servers to validate senders by requesting login credentials before allowing mail transmission.

  10. How does SPF prevent email spoofing?
    SPF verifies sender IP addresses against authorized IP ranges published in DNS records, preventing unauthorized use of domains in spoofed “From” addresses.

  11. What are MIME types in SMTP?
    MIME (Multipurpose Internet Mail Extensions) types specify encodings of message content like plain text, HTML, images, audio, video attachments.

  12. What is the difference between ASCII and MIME emails?
    ASCII only allows plain text characters. MIME enables rich text formatting and attachments by encoding binary data and multimedia.

  13. Can you explain how SMTP is vulnerable?
    Vulnerabilities include being exploited as open relays for spam, dictionary attacks on accounts, phishing attacks, denial of service attacks, spoofing/hijacking attacks.

  14. How does SMTP over SSL/TLS provide security?
    SSL and TLS encrypt the SMTP session, prevent interception or tampering with messages, and provide authentication mechanisms between client/server.

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