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The main difference between IMAP and POP3 is that POP3 protocol assumes that there is only one client connected to the mailbox, while IMAP protocol allows simultaneous access by multiple clients.

The main advantages of POP3:

- Mails load very quickly once they're downloaded.
- Mails are stored on your computer so they're not subject to server quotas.
- Widely supported by ISP's and email providers.
- Supported by all email clients.

The main disadvantages of POP3:

- Mails are stored on your hard drive so they're not accessible from other computers.
- Sent mail and drafts are not accessible from other computers either.
- Mails are eventually removed from the server.
- Mails are stored in files that are not compatible between different programs.
- Difficult to migrate mails from one program to another.
- Difficult to backup your emails.

The main advantages of IMAP:

- Mails are stored on the server are are accessible from any computer and webmail client.
- IMAP mail folders are synchronised with webmail.
- Sent emails and drafts are also stored on the server.
- Server-side spam filtering is easily implemented with IMAP.
- IMAP mail is backed up on the server and can be easily restored.
- Relatively easy to move and store mails from one client to another.
- Ability to read emails from multiple computers without having to copy mail files over.

The main disadvantages of IMAP:

- The first time emails are opened they are slower to load.
- Sensitive to storage quota limits and require archiving of old mails.
- Not all ISP's and email providers offer IMAP.
- Not all email programs support IMAP.