Duplicate emails in Sent Items when sending from a shared mailbox.

Duplicate emails can be present in Sent Items mailbox when sending through a shared mailbox, even though only one email was actually sent.  

Disabling the setting "MessageCopyForSentAsEnabled" should fix this issue as shown below.

Understanding the "MessageCopyForSentAsEnabled" Setting

The MessageCopyForSentAsEnabled setting is crucial for managing how sent emails are stored when using shared mailboxes in Microsoft 365. This setting determines whether a copy of emails sent as a shared mailbox is saved in the shared mailbox's Sent Items folder.

Where to Change the Setting

You can change the MessageCopyForSentAsEnabled setting in several ways:

1. Exchange Online PowerShell:
   • You can enable this setting using PowerShell commands. The command to enable it is:

Set-Mailbox -MessageCopyForSentAsEnabled $True

• This command ensures that emails sent as the shared mailbox are saved in both the sender's and the shared mailbox's Sent Items folders.
1. Microsoft 365 Admin Center:
   • Navigate to the Microsoft 365 admin center.
   • Go to Teams & groups and select Shared mailboxes.
   • Click on the specific shared mailbox, scroll down to Sent items, and click Edit.
   • Enable the options for Copy items sent as this mailbox and Copy items sent on behalf of this mailbox.
2. Registry Settings (for Outlook clients):
   • For Outlook clients, you can modify the registry to control how sent items are handled. The relevant registry key is:

HKEY_CURRENT_USER\Software\Microsoft\Office<version>\Outlook\Preferences

• You can create a DWORD value named DelegateSentItemsStyle to manage how sent items are stored.

Expected Behavior Based on Settings

Here's a summary of how the settings interact:

Conclusion

The MessageCopyForSentAsEnabled setting is essential for ensuring that sent emails from shared mailboxes are properly stored, allowing all users with access to the shared mailbox to see the sent messages. Adjusting this setting can be done through PowerShell, the Microsoft 365 admin center, or registry modifications for Outlook clients.

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