An e-mail message may not be delivered at the scheduled time when you enable the "Do not deliver before" option in an Outlook message

When you enable the Do not deliver before option to defer the delivery of a Microsoft Office Outlook e-mail message, the message may not be delivered at the scheduled time.

This behavior may occur if the following conditions are true:

• Outlook is running in Cached Exchange Mode.
• Outlook is not running at the scheduled time of the deferred delivery.

When Outlook is running in Cached Exchange Mode, it uses a local offline folder file (.ost). This file is periodically synchronized with the server. When you enable the Do not deliver before option to defer the delivery of a message, the message is deferred to the local Outbox, not to the server. Therefore, the message will not be delivered from the local .ost file unless Outlook is running when the message is scheduled to be delivered. This behavior does not occur in when Outlook is running in online mode.

To work around this behavior, enable the Do not deliver before option to defer delivery of the message when Outlook is running in online mode.

This behavior is by design.