Mail transfer agents connect using standard SMTP.
This is server-to-server delivery, not implicit TLS or submission.
Which standard port is used?
SMTP server-to-server delivery uses TCP port 25. Client submission commonly uses a different port such as 587.
Detailed explanation
Incorrect. 22/tcp is SSH.
Incorrect. 22/tcp is SSH.
Incorrect. 53 is DNS.
Incorrect. 53 is DNS.
Incorrect. 110/tcp is POP3.
Incorrect. 110/tcp is POP3.
Correct. TCP port 25 is SMTP's standard server-to-server port.
Correct. TCP port 25 is SMTP's standard server-to-server port.
Try it yourself
An example you can run in a temporary verification environment.
getent services smtp | sed -n '1p'Expected result
smtp 25/tcpを示すサービスデータベース行Key points
- SMTP is 25/tcp
- POP3 is 110/tcp
- Submission has a different purpose
Notes
- Environment: Linux NSS servicesデータベース / 外部通信なし
- Command output formatting can vary slightly by distribution or tool version.
- Run the example in a temporary directory or process when possible.
Foundation review
Read the scope first
Check whether the command acts on the current shell, a new process, an existing process, or a file.
Verify the observable result
Use the supplied command and compare the output with the expected result.