What are POP & IMAP?
POP and IMAP are internet protocols used by mail clients (such as Microsoft Outlook, Thunderbird or your mobile phone’s mail app) to retrieve messages from our servers. We recommend using IMAP.
Comparison
POP
This protocol only allows you to access your Inbox, and all the email you download is stored locally on your computer or device. Consequently, your email may only be visible in that particular mail program, rather than across devices or in webmail.
IMAP
IMAP allows you to access any folder in your email account and it essentially syncs messages across all devices. Any changes you make to your email or folders in your email software will be visible when you next sign in to webmail.