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N10-005 · Question #405

A network administrator is tasked with blocking unwanted spam is being relayed by an internal email server. Which of the following is the FIRST step in preventing spam that is originating from bots…

The correct answer is A. Closing off port 25 on the firewall. Closing outbound port 25 on the firewall is the first step to stop bots from directly sending spam using SMTP, without disrupting legitimate mail server operation.

Network security

Question

A network administrator is tasked with blocking unwanted spam is being relayed by an internal email server. Which of the following is the FIRST step in preventing spam that is originating from bots on the network?

Options

  • AClosing off port 25 on the firewall
  • BClosing off port 53 on the firewall
  • CTurning off the SMTP service on the email server
  • DTurning off the IMAP service on the email server

How the community answered

(36 responses)
  • A
    72% (26)
  • B
    17% (6)
  • C
    3% (1)
  • D
    8% (3)

Why each option

Closing outbound port 25 on the firewall is the first step to stop bots from directly sending spam using SMTP, without disrupting legitimate mail server operation.

AClosing off port 25 on the firewallCorrect

Port 25 is the standard SMTP port used to relay email. Bots on the internal network use port 25 to send spam directly to external mail servers. Blocking outbound port 25 at the firewall prevents bot traffic from leaving the network while allowing the legitimate internal mail server to continue handling mail through controlled relay channels.

BClosing off port 53 on the firewall

Port 53 is used by DNS for name resolution; closing it would break all hostname lookups on the network and has no effect on SMTP-based spam relay.

CTurning off the SMTP service on the email server

Turning off the SMTP service entirely would stop all outbound email from the mail server, preventing legitimate business email in addition to spam.

DTurning off the IMAP service on the email server

IMAP (port 143) is an inbound mail retrieval protocol used by clients to read email; disabling it has no impact on outbound spam relay by bots.

Concept tested: Blocking outbound SMTP port 25 to prevent bot spam relay

Source: https://learn.microsoft.com/en-us/exchange/mail-flow-best-practices/how-to-set-up-a-multifunction-device-or-application-to-send-email-using-microsoft-365-or-office-365

Topics

#SMTP port 25#spam prevention#email security#firewall configuration

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