IMAP and POP are email messaging protocols that have been around for over 15 years. These protocols are typically used to access email on legacy client devices, like mobile devices or client computers that do not or cannot have Microsoft Office Outlook installed. In many cases, these devices can use Microsoft Office Outlook Web Access to access their email.
In other cases, you may need to be able to download and have an offline copy of the email. These are the times that IMAP or POP clients should be used. Neither of these protocols is actually used to send email, so they need to be used in conjunction with the SMTP protocol.
The main difference between IMAP and POP is that IMAP has support to synchronize mail with the server and organize messages into folders, where POP can only retrieve email from the server. This makes IMAP a more suitable solution for mailboxes connected with multiple clients, as the email along with information about whether the messages has been read or moved to another folder will remain on the server for retrieval by other clients.
POP3 clients can be configured to leave a copy of the messages on the Exchange server; however this does not indicate whether the mail has been read from another client or if it has been deleted on another client.
From the Exchange perspective, configuring the protocols is very similar. You need to enable then start the services using the Services snap-in. Next, you need to configure a TLS certificate in order to provide encryption for the authentication and messaging information, as well as the URL for calendaring information. Lastly, you need to configure the SMTP virtual server to accept email from the POP or IMAP clients. This is usually done by configuring IP and authentication restrictions on a separate SMTP virtual server from the one that is used to send and receive email to the Internet.
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This was first published in April 2010