When you send an email message from a shared mailbox, the sent email message remains in your Outbox until you manually perform a Send/Receive operation.
This problem occurs when all the following conditions are true:
Your Outlook profile is configured in online mode (not cached Exchange mode).
You have the DelegateSentItemsStyle registry value set to 1.
You can resolve this issue by using one of the following methods. If these methods are not acceptable, see the Workaround section for additional options.
Method 1: Change the value of DelegateSentItemsStyle to 0 in the registry.
If you set DelegateSentItemsStyle=0 in the registry, email messages that you send from a shared mailbox will be copied to your Sent Items folder and not the Sent Items folder of the shared mailbox.
Important Follow the steps in this section carefully. Serious problems might occur if you modify the registry incorrectly. Before you modify it, back up the registry for restoration in case problems occur.
Start Registry Editor. To do this, use one of the following procedures, as appropriate for your version of Windows.
Windows 8 and Windows 10: Press Windows Key + R to open a Run dialog box. Type regedit.exe and then press OK.
Windows 7 or Windows Vista: Click Start, type regedit in the Start Search box, and then press Enter. If you are prompted for an administrator password or for confirmation, type the password, or click Allow.
Windows XP: Click Start, click Run, type regedit, and then click OK.
Locate and then select the following registry key:
Note In this key path, the placeholder x.0 represents 16.0 for Outlook 2016, 15.0 for Outlook 2013, 14.0 for Outlook 2010, 12.0 for Outlook 2007, and 11.0 for Outlook 2003.
Select the DelegateSentItemsStyle value.
On the Edit menu, click Modify.
Type 0, and then click OK.
Exit Registry Editor.
Method 2: Change your profile to cached mode
If you must have the functionality that is offered by the DelegateSentItemsStyle registry value (=1), your other option is to change your profile from online mode to cached mode.
Outlook 2010 and later versions
On the File tab, click Account Settings, and then click Account Settings.
On the E-mail tab, select your Exchange Server account, click Change, and then enable the Use Cached Exchange Mode option.
Click Next, and then click Finish.
Exit and restart Outlook.
On the Tools menu, click Account Settings.
On the E-mail tab, click the Exchange Server account, and then click Change.
Under Microsoft Exchange server, click to select the Use Cached Exchange Mode check box.
Exit and restart Outlook.
On the Tools menu, click E-mail Accounts.
In the E-mail Accounts dialog box, click View or change existing e-mail accounts, and then click Next.
Select the Exchange Server account, and then click Change.
In the E-Mail Accounts dialog box, click to select the Use Cached Exchange Mode check box, and then click Next.
Click OK, and then restart Outlook.
If the methods provided in the Resolution section are not acceptable, and if your mailbox is located on Microsoft Exchange Server 2010 Service Pack 2 Update Rollup 4 or later version, or Exchange Server 2013 Cumulative Update 9 or later version or Office 365 Exchange Online, the Exchange administrator can configure similar behavior on the server. These configurations do not require the DelegateSentItemsStyle registry key to be enabled, therefore the issue will not occur.
Microsoft Exchange Server 2010 Service Pack 2 Update Rollup 4 or later:
Update Rollup 4 for Exchange Server 2010 Service Pack 2 introduced a new Exchange PowerShell cmdlet to configure the Sent Items folder to which a message is copied. Because this new feature is handled by the Exchange server, Outlook can be configured for online or cached Exchange mode. However, the Exchange server feature works only if the Outlook DelegateSentItemsStyle registry value is disabled.
For more information about the Set-MailboxSentItemsConfiguration cmdlet, click the following article number to view the article in the Microsoft Knowledge Base:
2632409 Messages sent by using the "Send As" and "Send on behalf" permissions are only copied to the Sent Items folder of the sender in an Exchange Server 2010 environment
Microsoft Exchange Server 2013 Cumulative Update 9 or later or Microsoft Office 365 Exchange Online:
Cumulative Update 9 for Exchange Server 2013 introduced a new feature which allows administrators to configure the Sent Item folder to which a message is copied. Because this new feature is handled by the Exchange server, Outlook can be configured for online or cached Exchange mode. Please note that if you enable this feature and enable the DelegateSentItemsStyle registry value with a cached Outlook profile at the same time, two copies of the sent item will be saved in the shared mailboxes Sent Items folder.
For more information about this feature, go to the following blog:
Exchange Blog - Want more control over Sent Items when using shared mailboxes?
The DelegateSentItemsStyle registry value is stored in the following location in the Windows registry: