POP vs IMAP / Choosing the type of e-mail account in Outlook etc.

IMAP/IMAP4 ((recommended)
Pros: e-mails remain on the server and may therefore be accessed from any machine connected to the Internet, e.g. via Webmail. Handy if you are permanently connected and wish to access all of your messages from several places at once (from home, while on the move or away on a trip, etc) or where several users share one department e-mail address from different workstations. You will always have the same view of messages on the server, with each user seeing the same messages: if a message is deleted by one user, other users will no longer see the message. If it is moved to a folder on the server, all users will see the message in that folder.
Cons:: in principle, you need to be connected to the Internet to read your messages (but some e-mail clients on some mobile devices will allow you to view the inbox even without a 3G/4G or wi-fi connection).

POP/POP3
Pros: e-mails are removed from the server and downloaded to the computer. They may therefore be read off-line. Handy if your connection is costly or not permanent.
Cons: e-mails no longer appear in Webmail. There cannot be several people accessing the same mailbox, since as messages are deleted each time the mailbox is checked, messages will appear to be missing or disappear. Moreover, if you "keep a copy of messages on the server" (one of the options in your e-mail client), as the number of duplicate e-mails stored grows, the greater the risk that you will experience problems. More information

More information

Since IMAP is not compatible with POP3, you should not attempt to log into Webmail while an e-mail client configured to use POP3 is open.

To switch an e-mail address from one protocol to the other, you must re-create an identical account in your e-mail client then copy the messages over. More information