What does Post Office Protocol (POP3) do with emails?

Post Office Protocol, specifically version 3 (POP3), is designed primarily for retrieving emails from a mail server. When a user connects to their email account using POP3, the server typically downloads the emails to the user's local device, such as a computer or smartphone. Once this transfer is complete, POP3 generally removes those emails from the server, which is why the correct answer is that POP3 deletes them once downloaded.

This characteristic of POP3 is particularly useful for users who prefer to have all their emails stored locally and accessed offline. However, it's important to note that some configurations allow emails to be left on the server, but that behavior is not the default for POP3.

The other options involve functionalities that POP3 does not provide. For instance, storing emails indefinitely or transferring them to another server does not align with how POP3 operates. Additionally, forwarding emails to a mobile device is not a feature of POP3, as its main function is simply to download emails to a device rather than sending or redirecting them in dynamic ways.