POP (Post Office Protocol) is a protocol for receiving (downloading) emails from a mail server. Using this email protocol, emails are downloaded and saved locally on the user's device. By default, a copy of the e-mail server is not saved, but most e-mail clients have the option to enable this setting.
The most used version of this protocol is v.3 - POP3.
All emails are downloaded to the Inbox. When you create additional e-mail folders for your e-mail account, they are created in the client program, not on the e-mail server. Folders, as well as their contents, are not stored on the mail server. With the POP3 protocol, sent messages are also stored locally on the device from which they were sent.
When setting up an email client, it is advisable to set the use of the newer and better mail verification protocol - IMAP - instead of POP3.