You may be able to identify the IP addresses and machine names for the firewall, and the names of internal mail servers by:

A.
Sending a mail message to a valid address on the target network, and examining the header information generated by the IMAP servers

B.
Examining the SMTP header information generated by using the -mx command parameter of DIG

C.
Examining the SMTP header information generated in response to an e-mail message sent to an invalid address

D.
Sending a mail message to an invalid address on the target network, and examining the header information generated by the POP servers

Explanation:
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