IMAP and POP are different protocols used by email clients for retrieving and storing emails.
IMAP (Internet Messaged Access Protocol)
IMAP is a protocol set to sync directly with the server, enabling you to read email. When email is accessed on a device (computer via email client, mobile device via application etc.), it temporarily caches a certain amount of email headers to be held locally until the full message is opened and it reconnects to the server to download the entire message. Changes made through a device, e.g. deletion of email, should show across all devices once properly synchronized.
The advantages of using IMAP are that you can access email seamlessly over several devices, and is recommended for anyone who intends to do this. The largest problem with this is in regard to the amount of email kept on the server, as quotas can be reached and mail will be refused if this is the case. For a fully efficient service on IMAP, constant Internet access is preferable.
POP (Post Office Protocol)
POP will connect to your server and download mail in full to the device which first access it, removes it from the server and closes the connection. Once properly accessed from one device, the message won't be available on any other devices.
Once downloaded, you can access your emails using POP if you are without internet access. Using POP will also mean that issues with mailboxes being full should no longer occur. POP causes issues when you are trying to access from more than one device, as the email will no longer be present on the server.
Can I use different protocols on different devices?
While this is physically possible, it is strongly recommended against as it is likely that you will experience problems with missing emails. In the case of missing email, one of the first steps in troubleshooting is to confirm that only one protocol is being used.
Which protocol is best for me?
• You are using multiple devices to access email
• Local storage is a problem
• Local backups are unavailable/unreliable
• You only access mail from one device
• Email access is required when offline
• You are concerned about quotas on your mailboxes