Simple Mail Transfer Protocol (SMTP) is an Internet standard for electronic mail (e-mail) transmission across Internet Protocol (IP) networks. A user has set up filtering on an Internet router. The local SMTP server has stopped sending or receiving messages from the Internet.
Why?

A.
The Internet router is blocking port 110.

B.
The Internet router is blocking port 25.

C.
The proxy server is filtering mail requests.

D.
The DHCP server is down.