When setting a new mail client, you will be given the choice between two options during the setup phase: IMAP or POP3.
Both IMAP and POP3 are protocols used to handle incoming mail. If someone sends you an email, your server receives and delivers that email to your email client based on the protocol you have selected, being either IMAP or POP3.
We will explain what each one is, giving you the information necessary to make the choice that much easier, based on your personal needs.
POP3 (Post Office Protocol), like its name suggests, treats your inbox as a post office. Any incoming mail is downloaded locally on your computer.
When you access emails via POP3, a copy of the email is stored locally on your computer and in most cases (but not all), the email is removed entirely from the mail server. This means that if you access your email on a device such as a desktop, that email will not be accessible on a different device such as a mobile as it is locally stored on your desktop already.
Most email clients allow you to keep a copy on the server, allowing you to download it locally on multiple devices. In this configuration, emails stored locally on two separate devices are treated as two separate emails. They are not synced. This also applies to any mail folders you have in place on one device as opposed to another. They will not replicate over.
POP3 works well for users that access email from one device. It also frees up email storage space as the mail server is constantly removing emails as you access them and store them locally on your device.
IMAP (Internet Message Access Protocol) works in the opposite way. IMAP lets you access email from multiple email clients or devices, as the email is stored and kept on the mail server directly.
When you access your email, you are essentially using your email client as a tool to directly access mail stored on your mail server. This allows for syncronisation between multiple devices and email clients. This means if you mark an email as read, or delete it, that action will be reflected across all your devices as you have done that task on the mail server directly.
IMAP is great for users who need flexibility when it comes to their email. However, It does mean your email server may fill up faster if not maintained properly.
What one should I use?
That question all depends on how you like to access your email.
If the benefit of email syncronisation and the option to access your email with an internet connection across multiple devices suits your needs, IMAP is the protocol for you.
If you prefer checking your email from one device and want all your emails (including attachments) accessible even without an internet connection, POP3 is a great option. Please be aware that unless you have your mail server configured to keep a copy of your emails on the mail server, your email will be gone if your device becomes damaged or corrupted.