Final answer:
Spammers and hackers look for open SMTP servers to send email through a network.
Step-by-step explanation:
Spammers and hackers primarily look for open SMTP servers when they want to send email through a network. SMTP stands for Simple Mail Transfer Protocol, and an open SMTP server allows anyone to use it to send emails, including spammers and hackers. By hijacking an open SMTP server, they can send large quantities of spam or even use it as a launching point for more sophisticated cyberattacks.
While open web servers, open POP3 servers, and open FTP servers may also be targeted by hackers for different purposes, they are not specifically used for sending email through someone else's network.