Outlook 2003 Modes and Oversized Messages

When Microsoft Office Outlook 2003 is running in Cached Exchange Mode and a user tries to send a message that's larger then the maximum size allowed for the mailbox, what happens?

This is a fascinating question, and I ended up going to Microsoft Product Support Services' (PSS's) Dave Mills to get the answer. The default behavior for Outlook 2003 Service Pack 1 (SP1) and Exchange Server 2003 SP1 is as follows:

  • When Outlook is running in online mode, Outlook immediately streams attachments to the server as the user adds them to a message. However, Outlook doesn't submit the message to the server until the user clicks Send. At that point, the server checks the size of the complete message, determines whether it's over the mailbox's message size limit, and if so, returns an error to Outlook. Exchange doesn't create a nondelivery report (NDR) because it never accepted the message for delivery.
  • When Outlook is running in Cached Exchange Mode (or offline mode), Outlook uploads the entire message—body and any attachments—to the server when a connection becomes available. At that point, the server determines whether the message is over the size limit but can't reject the message, which has already been submitted. Therefore, the server has to generate an NDR that has the original message attached.

This behavior is obviously inconsistent. If users sometimes use Cached Exchange Mode and sometimes use online mode, they're probably wondering why Outlook behaves differently for no apparent reason. To get Outlook to respond consistently regardless of the mode it's running in, you can install two hotfixes: one for Exchange 2003 (see the Microsoft article "A message that exceeds the configured size limit is sent to a server that is running Exchange Server 2003" at http://support.microsoft.com/?kbid=894795) and one for Outlook 2003 (see the Microsoft article "Description of the Microsoft Office Outlook 2003 post-Service Pack 1 hotfix package: May 2, 2005" at http://support.microsoft.com/?kbid=897247). When you install both hotfixes, Outlook behaves the same in all modes: When a user submits a large message, Outlook queries the server for its submission size limit before submitting the message. When the message is over that size limit, Outlook generates an error, and the message remains in the user's Outbox.

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