220-1101 Question 93
Single answerMail serversA technician is setting up email on a user's new smartphone. The user says outgoing mail works when connected to the office Wi-Fi, but sending fails when using a cellular connection. Incoming mail works in both locations. The email account was manually configured with the company mail server names. Which of the following is the MOST likely cause?
- A
The outgoing mail server is configured to use SMTP, and the cellular provider is blocking or restricting the required SMTP port
- B
The incoming mail server should be changed from IMAP to POP3 so messages can be sent from outside the office
- C
The DNS server on the phone cannot resolve MX records, preventing the device from sending email
- D
The email account must use LDAP before the smartphone can authenticate to the mail server
Show answer and explanation
Correct answer: A
Explanation
This scenario tests practical knowledge of mail server roles and common connectivity issues. Incoming mail protocols such as IMAP and POP3 are separate from outgoing mail via SMTP. Because the user can receive mail on both networks, the mailbox itself is reachable and the account is at least partially configured correctly. The failure only when using cellular strongly suggests an SMTP submission issue related to the network or port configuration. Industry best practice is to use authenticated SMTP submission on port 587 with STARTTLS rather than legacy port 25, which many ISPs and mobile carriers block to reduce spam. This aligns with common provider documentation from major email services and general email best practices published in IETF standards for message submission, such as RFC 6409.
- A. Correct.
Correct. Outgoing email uses SMTP. A common real-world issue is that sending works on one network but fails on another because the alternate network blocks or restricts SMTP traffic on traditional ports, especially TCP 25. Many mail systems instead require authenticated submission on TCP 587, often with STARTTLS, or TCP 465 with implicit TLS depending on the provider. Since incoming mail works everywhere, the problem is most likely tied specifically to the outgoing SMTP configuration or the network path used for SMTP.
- B. Incorrect.
Incorrect. IMAP and POP3 are protocols for retrieving email, not for sending it. Changing from IMAP to POP3 would not fix an outbound mail problem. This distractor reflects a common misconception that any mail protocol setting affects all email functions equally.
- C. Incorrect.
Incorrect. MX records are used by mail servers to locate the destination mail server for domain-to-domain delivery. End-user mail clients typically send outgoing mail to a configured SMTP server by hostname, using standard DNS A or AAAA lookups for that server. Failure to resolve MX records on the phone would not be the most likely explanation for why sending works on office Wi-Fi but not on cellular.
- D. Incorrect.
Incorrect. LDAP is used for directory services, such as centralized address books or authentication in some enterprise environments, but it is not a required protocol for standard email sending from a smartphone. A phone can authenticate directly to an SMTP server without LDAP in many normal configurations.