Both protocols serve the same purpose of accessing mailbox content.
POP3 (Post Office Protocol)
It is an internet protocol used for downloading messages from the mailbox on the server to a local mail client. All messages from the inbox folder are downloaded. In the mail client, you can set it to download either just the headers (this allows you to mark messages for download or deletion after previewing them) or the entire messages. After downloading, messages can either be left on the server, deleted after X days from the download (note: not from the delivery), or deleted only after being removed from the trash. The POP3 protocol does not support full message synchronization across multiple devices.
- No internet connection is required to work with downloaded messages.
- If messages are deleted immediately after download, there is no risk of filling up the mailbox.
- If set to leave messages on the server (even for a certain period), messages deleted (even accidentally) from the Inbox won't be removed from the server mailbox.
- Downloaded messages are only available to a single mail client; incorrectly configured clients may delete messages from the server after downloading, making them unavailable to other clients (e.g., IMAP).
- Sent emails or emails moved to other folders are not saved on the server mailbox.
- Depending on the frequency of checking for new mail, downloaded messages might have a delay in appearing in the mail client as "delivered."
- When messages are left on the server and accessed from multiple clients, there is no synchronization of message status (read/unread).
- If data files are damaged, disk crashes, or the computer is stolen, messages can be lost (backing up the email client data is recommended).
IMAP (Internet Message Access Protocol)
Unlike the POP3 protocol, IMAP displays exactly what is in the mailbox on the server (content - messages, folder structure - is mirrored). The protocol allows operations directly on the server (moving messages to other folders, deleting messages). It also allows adding message status information (read/unread/deleted) and synchronizing them across different mail clients. Additionally, it provides detailed mailbox information (available space in the mailbox) and enables searching for messages without needing to download them. IMAP is particularly suitable for mobile clients or scenarios where multiple users access the same mailbox or use various mail clients.
- Allows access from multiple devices (mobile phone, tablet, notebook, workstation).
- Emails sent from one mail client are accessible in other mail clients.
- New messages are usually notified immediately after delivery to the mailbox.
- Read messages on one device are marked as read on other mail clients as well.
- If data files are damaged, disk crashes, or the computer (or mobile device) is stolen, messages remain in the mailbox on the server.
- Active work with downloaded messages requires an internet connection.
- Deleting a message in one mail client reflects on all other devices and in the server mailbox (cleaning the mailbox might lead to the loss of all emails in that folder).
- Delivering and sending large messages may lead to filling up the mailbox.
Regarding saving space in the mailbox, it's useful to remember that email protocols were not primarily designed for sending large messages (messages with large attachments). A message with a 10MB attachment will have a larger size due to its processing during sending. It is more suitable to send a link to a file stored in a data repository.