Some mail servers allow users a certain amount of control over aspects of their mail account, such as passwords, mail aliases, descriptions, vacation messages, and forwarding options. At first glance, these options don't sound like much of a security threat, but because they can be used to falsify identities or intercept mail, the potential for damage is great. Perhaps they're trying to get sensitive information from one of your vendors, like your account number and the amount of open credit. Or perhaps they want to do some social harm by sending negative e-mail to others on the network. Whatever the reason, you can protect against it by using a mail server that can authenticate a connection before it's allowed to send mail.
Intruders also use mail servers to gain access to sensitive company information.
If they crack the password to one of your mail accounts, they may be able to forward mail destined for that mailbox to themselves instead, or they may even simply log on and read it from the server. The two best ways you can protect yourself against any type of mailbox hijacking are to encrypt all your e-mail and to practice good password selection and rotation policies. PGP was created by Phil Zimmermann and uses the RSA and IDEA algorithms, which make it one of the best ways to encrypt information. Another way to protect your POP3 server, if your POP3 server doesn't have this ability built in, is to install a packet filter or some other type of firewall and limit access to the POP3 server to only those within your own networks. You should be aware that intruders sometimes retaliate against others by bombarding their mail server with high volumes of mail or by sending mail with giant file attachments.