Difference between pop, imap and exchange
When using POP3 (Post Office Protocol, version 3), all of the messages are downloaded from the mailserver and saved locally. Your Email is only accessible from one computer/device and Incoming Mail is no longer available when using WebMail or any other computer/device (unless configured otherwise).
Mail always available on the computer/device for offline consultation.
Sent Items available locally ONLY (no copy exists at all times on the mailserver); Speed of mail download dependent on bandwidth (large attachments may take some time).
IMAP (Internet Message Access Protocol, currently version 4) has features found in both POP3 and Exchange protocols.
When using IMAP, your Inbox is stored on the mailserver whereas the Sent Items are still stored locally (unless otherwise specified). When you check your mail, your computer contacts the mailserver to show you the new Incoming Mail. All of your Inbox is available from any computer and you can check it from anywhere in the world by using Webmail.
Incoming Mail always available on multiple computers and/or Webmail.
Sent Items available locally ONLY (no copy exists at all times on the mailserver).
ActiveSync or Exchange
This protocol has been introduced by Microsoft. All the Email (Incoming and Sent Items) is stored on the central Exchange mailserver, where it can be checked using a capable mail client (such as MS Outlook or most current smartphones) or via webmail. You may have the option of storing messages in local folders locally for offline consultation (in Outlook this is referred to as "Cached Mode").
Exchange mode also permits syncing of Calendar, Contacts and Notes.
Please Note: sometimes (above all on older mobile devices) this protocol is referred to as ActiveSync.
Incoming Mail and Sent Items always available on multiple computers and/or Webmail (mail is synced between mailserver and client). Supported by most current mobile devices (tablets, pads, smartphones); Calendar, Contacts and Notes synced as well.
Not all computer mail clients do support or fully support MS Exchange Mode (in Windows only MS Outlook, in Mac OSX Apple Mail and MS Outlook). Unless configured otherwise, offline consultation not available.
Please Note: We strongly suggests to use Exchange protocol as it is an easy way to keep your mail backup and best option to check mail worldwide and from different computers/devices.