POP3 (Post Office Protocol 3) is the most common standard for internet email messaging. With POP3, new messages are downloaded to your email client software on a scheduled basis, and are available on your computer when both offline and online. Normal email operations such as marking a message as being read, deleting or moving an email message to another folder all occur at your local computer only - none of these changes are communicated back to the email server.
IMAP (Internet Message Access Protocol) differs from POP3 in that it's primarily intended to be an "online" protocol. While IMAP can be used both online and offline, every operation you perform on a message such as marking it as being read, deleting it or moving it to another folder is communicated back to the email server.
Pro's and Cons:
- The status of your email messages (e.g. read, deleted, replied to) is synchronized across all devices you use to access email (e.g. webmail, cell phone and desktop).
- The Inbox, Sent Items, Deleted Items, Junk Mail and other custom email folders are available in all devices used to connect to your account. (although sometimes sent items can be an issue on certain devices.)
- IMAP is slower
- IMAP requires an "always-on" internet connection to function as intended
- IMAP uses more data bandwidth
- Messages may be visible in your email client, but may not be readable if offline
- Some email clients have poor implementations of the IMAP protocol (noticably, Microsoft Outlook)
- When you delete an email, it doesn't just delete it from the computer (device) that you are working on, but it also sends a message back to the server to delete it from there as well. When that happens, the email becomes unavailable (deleted) on all of your devices simultaneously.
- Your mailbox online becomes larger as more mails are received, this can require a larger mailbox, or that you manually delete emails from your mailbox.
- POP3 is faster
- Messages are always available offline
- POP3 uses less bandwidth
- POP3 does not require an 'always on' internet connection to function as intended
- POP3 is well supported in all email clients
- POP3 can support multiple devices if you choose to leave mail on the server, however any pop3 accounts must be configured the same on all pcs, devices such as ipads, tabs and phones can just use imap so they all get to see the emails.
- Backing up of youe email data can be done locally to an external device.
- The status and location of your messages is not synchronized across multiple devices.
Deciding between the two
Ultimately, the decision of whether to use IMAP or POP3 to access your account comes down to how you would like to access your email. If you primarily access your email using a desktop email client such as Outlook, MacMail or Thunderbird and nothing else, POP3 is probably the best choice.
If you access your email using a variety of different devices (such as webmail, desktop mail and a mobile device), and require that the status of the messages are synchronized across all of these, IMAP is the better choice.
If you're still unsure, POP3 is probably the protocol you're accustomed to using, so choosing it will insure your experience is as you would normally expect.