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SMTP Roles in Email Delivery: When a Mail Server Sends Mail to Other Mail Servers

SMTP Roles in Email Delivery: When a Mail Server Sends Mail to Other Mail Servers

Verified Sources
Nov 3, 2026

In SMTP, the same physical mail server can take different roles within a session. In general, when a mail server sends (relays) mail to another mail server, it behaves as the SMTP “client” in that hop—because it initiates the SMTP connection and issues commands to the remote server. This corresponds to option (i) SMTP client. 3

A key idea is the client–server model is per SMTP session/hop: the initiating side is the SMTP client and the accepting side is the SMTP server for that conversation. 2

Quick role intuition (per hop)

Footnotes

  1. Simple Mail Transfer Protocol - Wikipedia - Describes SMTP as the standard protocol and that mail servers/MTAs use it to send/receive; notes the client (initiating host) and server (receiver) concept. ↩

  2. SMTP (Simple Mail Transfer Protocol) - Wikipedia - Explains that an SMTP session consists of commands from an “SMTP client” and responses from an “SMTP server,” and discusses hop/relay behavior. ↩ ↩2

  3. RFC 5321 – Simple Mail Transfer Protocol (PDF) - Defines SMTP procedures and describes sessions initiated by a client connecting to a server and then proceeding with SMTP procedures. ↩ ↩2

SMTP Explained (Client vs Server Roles)

What counts as “sending mail to other mail servers”?

When your mail transfer agent needs to hand off an email to a destination (or next-hop) mail server, it establishes a connection and performs the SMTP transaction with that remote server. In that transaction, the local system is the SMTP client, and the remote system is the SMTP server. 2

This is also reflected in common protocol descriptions: SMTP sessions consist of a client initiating the connection and issuing commands (e.g., EHLOEHLO, MAIL FROMMAIL\ FROM, RCPT TORCPT\ TO, DATADATA), while the server responds with status codes. 2

Key terms (for the MCQ)

  • SMTP: defines how SMTP conversations work.
  • SMTP session: per RFC 5321.
  • SMTP client: sender side for that hop.
  • SMTP server: receiver side for that hop.

Footnotes

  1. SMTP (Simple Mail Transfer Protocol) - Wikipedia - Explains that an SMTP session consists of commands from an “SMTP client” and responses from an “SMTP server,” and discusses hop/relay behavior. ↩ ↩2

  2. How SMTP Works: The Journey of an Email Message | eEvidence - Provides a practical overview: an outgoing server sends to the recipient’s server via SMTP and the remote server responds/accepts delivery. ↩

  3. RFC 5321 – Simple Mail Transfer Protocol (PDF) - Defines SMTP procedures and describes sessions initiated by a client connecting to a server and then proceeding with SMTP procedures. ↩ ↩2 ↩3

  4. Simple Mail Transfer Protocol - Wikipedia - Describes SMTP as the standard protocol and that mail servers/MTAs use it to send/receive; notes the client (initiating host) and server (receiver) concept. ↩

  5. RFC 5321 - Simple Mail Transfer Protocol | PMAIL COMMUNITY - Includes section text stating: an SMTP session is initiated when a client opens a connection to a server and the server responds with an opening message. ↩

Correct: (i) SMTP client

How the SMTP client–server role appears during mail handoff

  1. 1
    Step 1

    The mail server decides where to deliver next (e.g., based on routing like MX lookup) and has a message ready for transfer. 2

    Footnotes

    1. Simple Mail Transfer Protocol - Wikipedia - Describes SMTP as the standard protocol and that mail servers/MTAs use it to send/receive; notes the client (initiating host) and server (receiver) concept. ↩

    2. How SMTP Works: The Journey of an Email Message | eEvidence - Provides a practical overview: an outgoing server sends to the recipient’s server via SMTP and the remote server responds/accepts delivery. ↩

  2. 2
    Step 2

    It initiates a TCP connection and begins an SMTP session by waiting for the server’s greeting. 2

    Footnotes

    1. RFC 5321 - Simple Mail Transfer Protocol | PMAIL COMMUNITY - Includes section text stating: an SMTP session is initiated when a client opens a connection to a server and the server responds with an opening message. ↩

    2. SMTP (Simple Mail Transfer Protocol) - Wikipedia - Explains that an SMTP session consists of commands from an “SMTP client” and responses from an “SMTP server,” and discusses hop/relay behavior. ↩

  3. 3
    Step 3

    The initiating side sends EHLOEHLO, then the envelope commands (MAIL FROMMAIL\ FROM, RCPT TORCPT\ TO) and DATADATA. 2

    Footnotes

    1. RFC 5321 – Simple Mail Transfer Protocol (PDF) - Defines SMTP procedures and describes sessions initiated by a client connecting to a server and then proceeding with SMTP procedures. ↩

    2. SMTP (Simple Mail Transfer Protocol) - Wikipedia - Explains that an SMTP session consists of commands from an “SMTP client” and responses from an “SMTP server,” and discusses hop/relay behavior. ↩

  4. 4
    Step 4

    The remote system responds with SMTP status codes to each command and accepts or rejects the mail transfer. 2

    Footnotes

    1. RFC 5321 – Simple Mail Transfer Protocol (PDF) - Defines SMTP procedures and describes sessions initiated by a client connecting to a server and then proceeding with SMTP procedures. ↩

    2. SMTP (Simple Mail Transfer Protocol) - Wikipedia - Explains that an SMTP session consists of commands from an “SMTP client” and responses from an “SMTP server,” and discusses hop/relay behavior. ↩

  5. 5
    Step 5

    On success, the server queues/accepts the message; then the SMTP session ends. For the duration of the hop, the initiating side is the SMTP client. 2

    Footnotes

    1. SMTP (Simple Mail Transfer Protocol) - Wikipedia - Explains that an SMTP session consists of commands from an “SMTP client” and responses from an “SMTP server,” and discusses hop/relay behavior. ↩

    2. How SMTP Works: The Journey of an Email Message | eEvidence - Provides a practical overview: an outgoing server sends to the recipient’s server via SMTP and the remote server responds/accepts delivery. ↩

Role mapping during a single SMTP hop (conceptual)

The same mail system can be client in one hop and server in another.

Common exam traps

Pro Tip: Think “initiator”

For an SMTP hop, initiating the conversation = SMTP client (client sends commands), while the receiving responder = SMTP server (server sends replies). 2

Footnotes

  1. RFC 5321 – Simple Mail Transfer Protocol (PDF) - Defines SMTP procedures and describes sessions initiated by a client connecting to a server and then proceeding with SMTP procedures. ↩

  2. SMTP (Simple Mail Transfer Protocol) - Wikipedia - Explains that an SMTP session consists of commands from an “SMTP client” and responses from an “SMTP server,” and discusses hop/relay behavior. ↩

Pro Tip: Don’t confuse server *identity* with server *role*

A mail server can be the SMTP client when sending to a remote server, and later become the SMTP server when accepting an incoming connection from another server. The role is per SMTP session/hop. 2

Footnotes

  1. SMTP (Simple Mail Transfer Protocol) - Wikipedia - Explains that an SMTP session consists of commands from an “SMTP client” and responses from an “SMTP server,” and discusses hop/relay behavior. ↩

  2. RFC 5321 – Simple Mail Transfer Protocol (PDF) - Defines SMTP procedures and describes sessions initiated by a client connecting to a server and then proceeding with SMTP procedures. ↩

Knowledge Check

Question 1 of 3
Q1Single choice

When a mail server sends mail to other mail servers using SMTP, it becomes which role?

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