IMAP and POP3 are different in how and for how long they store mail on the mail server.

POP3

When you use POP3, your mail server deletes messages when you download them. That means that you can only access messages from the computer that you used to download them. Also, POP3 does not require a constant connection while you read email. While this system conserves your mail server’s disk resources, it limits how your users can access their email. 

IMAP

When you use IMAP, your mail server permanently stores messages. This means that you can access them through any computer as long as you have the correct login information. Also, unless you use a mail client that synchronizes folders and caches messages, IMAP requires a constant connection while you read email. While this method is more convenient than POP3, this method generally requires more dedicated disk space than POP3 because users tend not to delete old email. However, if the users monitor their disk usage and delete old messages when necessary, IMAP is still viable on a mail server with limited resources. 

Web hosts and users generally prefer IMAP due to its convenience. Carefully consider your system’s available resources before you choose a courier.