POP3 and IMAP are two completely different ways of accessing your email from standalone mail reader programs such as Microsoft Outlook or Mozilla Thunderbird. Neither is "better" or "worse" than the other, and which you choose depends largely on your email usage habits.

POP3

With POP3 mail, when you check mail, all of the messages are downloaded onto your computer to be read by your standalone mail program, and then deleted from the server (though some programs have an option to leave mail on the server). Since the messages are stored on your computer, this means that searching the messages is substantially faster than the same search over IMAP. Since the mail is now stored on your computer instead of High5's mail servers, it means the messages do not count against your mail quota. Many popular mail programs such as Microsoft Outlook cannot filter IMAP mailboxes, meaning that if you have an existing set of filters defined in your favorite mail program to sort mail automatically as it is received, you can continue to use these filters if you receive mail using POP3.

Since mail is deleted from the server after being checked, it cannot be checked with a web browser or another computer once it has been downloaded. POP3 is ideal for situations where email is only read from one computer. Because many mail programs have several different file formats for storing messages, it may be difficult to import mail from one to another, especially from a less popular program to a more popular one. If you expect to be trying several different programs until you find one you like, we recommend using IMAP until you can settle upon one.

Note that while it is possible to leave messages on the server with many POP3 mail clients, occasionally bugs in the implementation of this feature may cause all of your mail to be downloaded several times, leaving you with hundreds or thousands of duplicate messages that cannot be easily removed. If you plan to leave messages on the server, it is strongly recommended that you connect using IMAP.

IMAP

IMAP is in many ways the functional opposite of POP3. Whereas POP3 downloads all mail to your computer, IMAP mail programs download only the headers (basic information about the message like the sender, the subject line and the date it was sent) of the message; the contents of a message aren't downloaded until you actually open that message.

Because your mail is stored on High5's servers, multiple computers can access a single account using IMAP at the same time, and web mail will continue to function after you have downloaded a list of messages.

Unless your mail program has an "offline mode" which downloads all of your messages, you may not be able to read your mail without an Internet connection. With a huge number of Internet subscribers moving towards "always-on" broadband connections such as DSL or cable, this may not be a problem for you, however, you may wish to consider this drawback when choosing your mail access method. Many programs will also allow you to copy your mail to "local folders" on your personal computer, so check the features of your program of choice before making a decision!

The biggest drawback of the IMAP format is that while many mail programs such as Mozilla Thunderbird have phenomenal IMAP support, some programs like Microsoft Outlook Express do not. They can occasionally stall when receiving or reading messages, requiring you to close and reopen the program. They may fail to download all headers correctly, and may fail to properly display certain messages.