IMAP and POP refer to the method (protocol) used by your email program to communicate with the server where your emails are stored.
If you are not sure which method to use refer to the following basic differences and choose the one that fits your needs.
IMAP Best suited for: Mobile phone, tablet, working from various locations or multiple devices.
The IMAP method, by default, allows the user to keep all messages on the server. It constantly synchronizes the e-mail program with the server and displays what messages are currently present. All the actions performed on the messages (reading, moving, deleting...) will be done directly on the server.
Because everything is kept on the server you are able to access email in the mailbox from any computer/device connected to the Internet. When using webmail via http://yourdomain.com.au/webmail (replacing yourdomain.com.au with your actual domain name you are accessing your mailbox via IMAP.
POP Best suited for: A single device, or one PC/Mac and a device. Home or single user.
The POP method, by default, is set to download all the messages from the mail server onto your computer/device. This means that all the actions performed on the messages (reading, moving, deleting...) will be performed on your computer/device.
Because everything is kept on your computer/device, you will not be able to reopen messages from any location other than the computer/device where the messages have been downloaded.
Once email has been downloaded it can be accessed only using the same computer/device.
* Some email programs have an option to setup the POP protocol to save the copy of the messages on the server after downloading them on the computer. For more information look in the instructions for setting up the e-mail client. This configuration will have to be set up on each computer where you wish to use this feature.
Important information when using IMAP
There is a quota (limit) on email storage, so it is necessary to regularly delete (and purge) unnecessary email messages from the server. When a mailbox is full it will not receive new email messages. Instead they will be returned to the sender (as a bounce) stating that your mailbox is full. Mail quotas are defined at the time you create the mailbox. If the quota is set to unlimited on any mailbox then you can use up to the entire quota of your web hosting account. To have mail delivery work again you must free up space in the mailbox. You can either remove unwanted mail, increase the mailbox quota via your account cPanel and/or increase the total quota on your web hosting account.