What are the 2 parts of SMTP?

What are the 2 parts of SMTP?

The two key parts that comprise the SMTP (Simple Mail Transfer Protocol) for email delivery are the envelope and content of the message. The envelope contains the routing information while the content holds the actual email message for the recipient.

What are the 2 parts of SMTP?

Overview of Parts of an SMTP Message

The core components that make up an SMTP message for transmission are:

  • Envelope – Specifies SMTP handshake, sender/recipient details, routing information.
  • Content – Comprises actual headers, body of email message.
  • Envelope encapsulates Content – Outer envelope wraps around inner message content like physical envelope around a letter.

So essentially the envelope facilitates SMTP transmission while content represents the real message to be delivered.

SMTP Envelope

The SMTP envelope frames the connection handshake and delivery routing information:

  • Establishes initial SMTP handshake and session between sending and receiving servers.
  • Sender details provided using SMTP MAIL FROM command.
  • List of recipients added with series of SMTP RCPT TO commands.
  • Determines routing path and relays message through multiple hops across mail servers based on recipient domain names.
  • Headers are added to envelope upon message submission summarizing transaction details.

So in essence, the SMTP envelope manages reliable transportation of the content using SMTP transmission protocol between mail servers.

SMTP Message Content

The content comprises the actual headers and body of the email message:

  • Structured using Internet Message Format (RFC 5322) with header fields and message body.
  • Contains From, To, Subject, Date and other headers describing message itself.
  • Body holds the actual email content – text, attachments etc.
  • Typically produced by user’s Mail User Agent like email clients or webmail interface.
  • Handed over to SMTP server only at stage of message submission.

So the content represents the real payload in form of the email while the envelope handles the transportation across SMTP.

Relationship Between SMTP Envelope and Content

The envelope and content have the following relationship:

  • Envelope encapsulates the content as its payload for transportation between source and destination mail servers.
  • Envelope headers contain metadata and instructions for routing via SMTP.
  • Content headers describe the actual message itself.
  • Servers only process the envelope. Content remains untouched during relay hops.
  • Information in envelope can differ from content headers. Envelope takes precedence for delivery.
  • Content signatures like DKIM are added to message body before enveloping.

So the envelope wraps the content payload to provide a transit coating that directs message forwarding between SMTP servers.

How the SMTP Envelope Facilitates Message Transport

Following mechanisms involving the envelope enable reliable SMTP message transmission:

  • SMTP handshake – Exchange of SMTP greeting, initiation of session.
  • MAIL FROM – Specifies sender address in envelope for transaction records.
  • RCPT TO – Adds list of recipient addresses into envelope for determining delivery path.
  • Envelope headers – Records timestamp, sizes, relay paths etc. for diagnostics.
  • SMTP commands – Envelope manages handshakes, transactions based on commands like DATA, EHLO, RSET etc.
  • Error handling – Envelope manages bouncebacks, failure notifications etc. in cases of delivery issues.
  • Server-to-server – Envelope enables direct server-server transfers shielded from user agents.

So in essence, the SMTP envelope handles all coordination needed for reliable hop-by-hop routing of the content payload between mail servers using SMTP.

Key Differences Between Envelope and Content

Envelope Content
Created by Mail Submission Agent and Mail Transfer Agent. Created by Mail User Agent (email software).
Comprises SMTP dialog, transmission metadata. Comprises headers and body of actual email message.
Stores sender and recipients needed for directing transmission path. Stores display From, To, Cc, Bcc email addresses.
Records application data like timestamps, unique message ID etc. Subject, date and message contents.
Determines routing of message between source and destination mail servers. The actual email payload/letter to be delivered.
Outer technical wrapper to facilitate transport over SMTP. Inner payload containing true headers and content.
Unchanged during relay hops between mail servers. Remains untouched during SMTP transmission process.
SMTP server acts only on envelope. Content remains opaque to it. SMTP server does not process content portion.

So in summary, the envelope routes the content payload between servers using SMTP while shielding the content from intermediate hops.

Reasons for Separation Between Envelope and Content in SMTP

There are several key reasons for envelope and content separation in SMTP:

  • Senders and receivers are hidden during relay hops for security.
  • Envelope provides technical wrapper abstracting content details from SMTP data channel.
  • Allows inserting trace information in envelope improving diagnostics without altering content.
  • Envelope handles all key transmission functions like routing, retries, errors etc.
  • Content signatures embedded in message body prevent tampering.
  • SMTP servers parse and act only on envelope metadata for efficiency.
  • Standardized separation facilitates interoperability between diverse networks and systems.

So maintaining distinct split between envelope and content enables secure, resilient and uniform email transfers across Internet.

Use of Envelope vs Header From Addresses by Receiving Servers

Receiving mail servers use envelope address over From header for delivery:

  • Envelope MAIL FROM sender is authenticated during SMTP transaction. More trusted.
  • Envelope sender is actual point of origination while From header can be spoofed.
  • Spam checks and filters rely on validated envelope sender to build reputation.
  • Bouncebacks and delivery failure notifications (DFN) are sent back to envelope FROM address by server.

So while not visible in message received, receiving SMTP servers lean on the envelope address for processing for accuracy, security and compliance.

Key Takeaways on Parts of SMTP

  • The core components in an SMTP message are the envelope and content.
  • Envelope handles SMTP handshake, sender recipient details and routing of message between mail servers.
  • Content comprises the headers and body which form the actual email payload.
  • Envelope encapsulates content and enables reliable hop-by-hop message transmission over SMTP protocol.
  • Separation provides security, flexibility and interoperability in message transmission process between diverse networks.


The split between the envelope and the content within an SMTP email message enables secure and reliable transmissions by abstracting key routing information from the actual content payload. The envelope establishes the sender-receiver connection, determines transmission path and manages coordination instructions like retries and errors. The content travels untouched end-to-end inside this protective envelope shell as it traverses intermediate hops protected from tampering. This elegant bifurcation allowed SMTP’s distributed mail routing model to scale globally while shielding private details of mail and providing tools to administrators for diagnostics. The longevity of core SMTP owes much to its vision of envelope abstraction which fulfilled security and interoperability objectives admirably.


  1. What happens to email content during SMTP delivery?
    The content comprising email headers and body remains completely untouched as the opaque payload carried inside the envelope across SMTP server hops to destination.
  2. Can Email content be tampered with during SMTP transmission?
    No, the content cannot be tampered with during delivery over multiple SMTP hops thanks to the encapsulation provided by the envelope and inbuilt safety mechanisms.
  3. Why are SMTP envelope and content kept separate?
    Keeping envelope and content separate enhances security, flexibility and interoperability of message transmission between diverse networks by focusing SMTP coordination instructions in envelope only.
  4. Does the SMTP envelope protect email content?
    Yes, the envelope provides an encapsulation layer that shields email content from manipulation and hides identities as messages get relayed between intermediate mail servers.
  5. What information is stored inside SMTP envelope?
    The envelope stores technical metadata like transaction details, routing instructions, sender/recipient addresses needed for directing transmission over SMTP hops to destination.
  6. What happens to the SMTP envelope upon message delivery?
    Once the message is delivered, the SMTP envelope has fulfilled its purpose. The envelope technical details stay hidden and are discarded while only the content is stored at destination.
  7. Can I view SMTP envelope data?
    No, envelope metadata is not readily visible to end users normally. But email headers may display trace routes and SMTP IDs giving clues on transmission path.
  8. Does changing email content affect SMTP envelope?
    No, the SMTP envelope remains independent of content portion. Changing email content headers/body does not affect the technical envelope used for transmission.
  9. What takes precedence for email delivery – envelope vs headers?
    Mail servers rely on the validated envelope sender for routing. So if envelope FROM and header From differ, envelope address is considered authoritative for delivery.
  10. Why does email content appear differently than what I composed?
    This happens because you compose the content portion. The surrounding envelope handling transmission over SMTP is added later by your device’s mail client and receiving SMTP server.
  11. Can I specify delivery instructions using email body content?
    No, any delivery instructions like requesting read receipts must be specified using envelope SMTP headers only. Message body content is not processed for transmission by SMTP servers.
  12. Do recipients see the SMTP envelope?
    No, recipients only see the delivered message content comprising headers and body that were encapsulated and transported as payload inside the opaque SMTP envelope during transit.
  13. What is a trace header in email envelope?
    A trace header records details of intermediate mail servers traversed between source and destination servers. It helps diagnose transmission and deliverability issues when present.
  14. Can spam checks read my SMTP envelope?
    Yes, spam and abuse checks by recipient mail servers examine envelope sender validity which helps build domain reputation. The content remains hidden from such checks.
  15. What information does Return-Path header in envelope contain?
    The Return-Path header logs the validated envelope MAIL FROM address to which bouncebacks and delivery failure notifications should be sent in case of issues reaching recipients.
  16. How does adding recipients to Bcc hide them in envelope?
    The envelope only lists recipient addresses added in RCPT TO. So adding extra recipients to Bcc in message content prevents exposing them in the transmission envelope.
  17. Can I spoof the SMTP envelope sender?
    No, the MAIL FROM sender specified in envelope is validated through SMTP handshake so spoofing it fails. However From header in content can be spoofed more easily in poorly configured servers.
  18. Why is email content sometimes altered if envelope is intact?
    Occasionally content can get corrupted during encoding issues etc. even if envelope remains intact during SMTP relay hops and message technically reaches destination to be delivered.
  19. Can I encrypt my SMTP envelope for security?
    Yes, envelope encryption is possible by using the SMTP TLS extension causing overall SMTP traffic including envelope to be encrypted between hops preventing snooping.
  20. Where on internet does SMTP envelope transmission occur?
    The SMTP envelope manages server-server communications which take place between source, relay and destination mail servers over the open Internet via TCP port 25 or secured alternatives.

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