Which of the following describes a behavior of a mail relay server?

A mail relay server is designed specifically to receive and forward email messages between different email servers, often across the Internet. This function enables the transfer of emails from a sender's server to a recipient's server, even when they are on different networks. By facilitating this communication, it allows remote users to send and receive emails that might not be directly accessible otherwise. The ability to relay messages is essential for enabling email communication across diverse geographical locations and network boundaries.

In contrast, the other options describe functionalities that are not characteristic of a mail relay server. Connecting only to local users would imply limited functionality, while encrypting all email communication generally requires additional protocols beyond the basic operation of a mail relay. The management of spam filters also typically involves different systems or software setups, which may work in conjunction with a mail relay but is not a primary defining feature of its operation.