IMAP versus POP3: POP3 (Post Office Protocol) and IMAP (Internet Message Access Protocol) are two different email protocols. POP3 is the most common way that people receive their emails in software tools such as Outlook. 

POP is basically a flow through entity—it just passes on the information to you at your email program. IMAP on the other hand, is interactive with your email program. When the POP email server receives email it stores it on the server until you to request it. When you open your email program (e.g., Outlook) you request the email from the server by pressing the Send/Receive button. When the POP server receives your request for mail, it sends the entire message to your email program. Once you receive the email, the message is no longer stored on the server unless you specifically tell it to keep a copy.

IMAP allows you to download emails from the server to your email program however, the difference is that when you request your email from the server it sends a copy rather than sending the entire email. It keeps a copy on the server while keeping a copy on your computer.

IMAP is good when you have more than one user monitoring a mailbox (email address) or use more than one computer to check your emails, bad thing is it uses lots of bandwidth, fills up lots of space on your mail server and if you have a big mailbox, it is slow.

POP3 is fast and generally suits most people's needs.