220-1101 Question 112
Single answerMail exchanger (MX)A small company moved its email service from an on-premises mail server to a cloud email provider over the weekend. On Monday, users can browse the web and send email from mobile devices using the provider's app, but customers on the internet report that new email sent to employee addresses is bouncing back as undeliverable. You verify that the company's website is working and that internal network connectivity is normal. Which DNS record should the technician check first to restore inbound email delivery?
- A
MX record for the company's domain
- B
A record for the company's web server
- C
CNAME record for the www host
- D
PTR record for the firewall's public IP
Show answer and explanation
Correct answer: A
Explanation
The best answer is the MX record for the company's domain. In real-world email migrations, the most common cause of inbound mail failure after moving to a new provider is that the MX record still points to the old mail server or was never updated correctly. Mail servers on the internet query DNS MX records to determine the destination for incoming mail. By contrast, A and CNAME records typically affect web or host name resolution, and PTR records are mainly relevant to reverse DNS checks used for outbound email validation and anti-spam processes. Best practice during an email migration is to update MX records according to the new provider's documentation, confirm priority values if multiple MX records are used, and allow time for DNS propagation based on TTL settings. This aligns with standard DNS behavior described in common vendor guidance from Microsoft 365, Google Workspace, and ICANN/IETF DNS references.
- A. Correct.
Correct. The MX (Mail Exchanger) record tells other mail servers where to deliver email for a domain. If a company changes email providers and inbound mail starts bouncing, the first item to verify is that the domain's MX record points to the new provider's mail server and has been configured correctly. This is the most direct DNS setting affecting inbound email routing.
- B. Incorrect.
Incorrect. An A record maps a hostname to an IP address, commonly for services such as websites. Since the website is already working, the web server's A record is not the most likely cause of inbound email failures. Someone might choose this because email services also depend on DNS, but inbound mail delivery is controlled primarily by MX records, not the website's A record.
- C. Incorrect.
Incorrect. A CNAME record creates an alias from one hostname to another. While some services use CNAMEs for hostnames like autodiscover or branded service endpoints, a CNAME for www affects website name resolution, not the domain's mail routing. This distractor reflects the common misconception that any DNS alias issue can cause email delivery failures.
- D. Incorrect.
Incorrect. A PTR record provides reverse DNS, mapping an IP address back to a hostname. Reverse DNS can affect outbound email reputation and spam filtering, but it is not the first record to check when outside senders cannot deliver mail to the company's domain. This option is plausible because DNS and mail are related, but PTR does not determine where inbound mail is sent.