It's one more issue that we need some help from Microsoft to identify and try make the easy as possible to end users. You will see during the description below
- Some end users cannot send emails because their Outbox Folders (when view with cache mode disabled) has more than their Default limit size and their messages looks like duplicated. Messages were already sent but still being archived under Outbox folder.
- This symptom has been increased the number of the user impacts since they were migrated to Exchange 2010 Mailbox.
General recommendations to narrow down the causes as follows:
- Verify the users have only one Outlook client open for their mailbox. If they have multiple workstations with Outlook open, instruct them to only have one open at a time.
- Verify if the affected users are on Outlook 2007 or Outlook 2010
- For users on Outlook 2007, educate users in Cached mode to avoid: known causes and help them to change their behaviors to prevent them from triggering the failure:
- Avoid Rules on items as they are sent
- Avoid moving items you have sent without waiting a significant amount of time (5 minutes or more)
- Do not have your Outlook/Exchange profile configured to deliver mail to a PST
- Do not select the option “In folders other than Inbox, save replies with original message”
- Even if users are updated to Office 2010, they still need to avoid having their mail delivered to PST or they can see similar symptoms
- Go ahead and deploy the SendOne reg key for Outlook 2007 users. This is the KB948984 you already have noted below and will help mitigate many of the known issues in that version.
Used the article and apply the registry 'SendOne' = 0 and restart the Outlook. After applying the SendOne reg key to your Outlook 2007 users, continue periodically reviewing the Mailbox Folder Statistics for the Outbox folder counts to verify the counts are no longer growing.
Outlook 2007 attempted to optimize the submission process with Exchange and it left some openings for certain client/user behaviors to interrupt that optimization. The “SendOne” reg key reverts to Outlook 2003 and prior cached behavior and removes the submission optimization.
PS: After deploying the registry entry for end users you should instructed the end users to cleanup their Outbox. Probably it will not be an easy way.