Most email software/apps allow mailboxes to be accessed via either POP3 or IMAP4.
These are two different methods of collecting email . On your desktop software/app, POP3 is downloaded directly from the mail server onto your computer/device where it will store a “local copy” of the mail, and usually delete it from the server. You can create local folders on your computer/device to organise the mail, but you won’t see these or your mail if you logged in to the email server using webmail.
If you set your account up as IMAP4, a copy of your emails are left on the server (where it will continue contributing towards your mailbox limit). The emails will stay on the server until you delete them from your email computer/app. IPAM4 accounts basically sync the email on the email server with your devices.
If you’re accessing your email on several different devices, you must use the same format on all devices – IMAP is recommended.