Executive Summary:

Go behind the scenes and learn why Outlook saves incoming POP3 email to the local .ost file first, and then synchronizes the .ost to the Exchange Server mailbox.


Within an Outlook profile, you can configure multiple email accounts. Up to and including Microsoft Office Outlook 2007, only one of these accounts could use MAPI access to an Exchange Server. However, you can add other accounts using different Internet protocols, namely POP3, IMAP4, and even HTTP. I don’t know of a documented maximum number of email accounts allowed in a single Outlook profile, but performance can degrade with multiple active accounts through memory demands, bandwidth requirements, and disk I/O contention. I have four accounts in my main profile–two POP3 accounts, an IMAP account, and an Exchange account–and the negative performance impact within Outlook and with other programs in use concurrent to Outlook, especially those accessing remote resources over my wireless connection, is sometimes quite tangible.

Post Office Protocol, best known as POP3, is a client access protocol as defined in RFC 1939. An email client, like Outlook, polls a POP3 server with valid credentials, to identify and download new messages from a mailbox. Exchange Server is able to function as a POP3 server, but not as a POP3 client (though many third party software products try to make it so). When Outlook is configured with one or more POP3 accounts with delivery to an Exchange mailbox, it's Outlook that retrieves the mail. As new content is pulled from the POP3 server, Outlook then directs it to the Exchange Server mailbox. If Outlook is using Exchange Cached Mode, then Outlook first saves incoming POP3 mail to the local .ost file, and then synchronizes the .ost file to the Exchange Server mailbox.

I confirmed this behavior for Microsoft Exchange Server 2007 and Outlook 2007 configured with a Cached Mode Exchange account and a POP3 account with the Exchange mailbox as the delivery location. By watching the .ost file size and Exchange mailbox size in real time, as well as doing packet captures during a POP3 session between Outlook and my ISP’s POP3 server, I was able to verify that Outlook saves the POP3 messages locally first. If Microsoft had Outlook redirect incoming POP3 content to the mailbox on the Exchange server first, the content would be making an unnecessary round trip from Outlook to the Exchange mailbox and back again to Outlook’s .ost file.

I have seen this multiple account configuration at an Exchange customer where they had acquired a small company that used Outlook with POP3 from a Windows-based email server. The employees from the acquired company were given Exchange mailboxes with POP3 access to their old server which still received new email during a transition period. It works well, even with Cached Mode. There may be times where you want to control whether POP3 accounts should be added to users’ profiles or perhaps you might just want to control when POP3 servers are polled for new mail. For more on those options see my tips “Using Send/Receive Groups in Outlook,” InstantDoc ID 101936 and “No Retroactive Prevention of New Account Creation,” InstantDoc ID 101934.