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When the Accounts Settings window opens, select the email account which fails to sync, and click the Repair button. I mean if Outlook is sending the same email twice, thrice or multiple times then “How will manage duplicate emails in Outlook’s sent or outbox mail-folder”. In this situation, contact your administrator to have this group policy removed.This works in most cases, where the issue is originated due to a system corruption. If you are unable to re-enable this setting because it is grayed out, then the setting is being managed by group policy. In the Send/Receive section, enable Send immediately when connected.In the Options dialog box, select the Mail Setup tab.In the Send and receive section, enable Send immediately when connected.In the Outlook Options dialog box, select Advanced.Use the following steps to re-enable the Send immediately when connected option. In the above registry key paths, represents the Outlook version (16.0 = Outlook 2016, Outlook 2019, or Outlook for Office 365, 15.0 = Outlook 2013, 14.0 = Outlook 2010, 12.0 = Outlook 2007, 11.0 = Outlook 2003). Policy key: HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\Outlook\Options\Mail
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Key: HKEY_CURRENT_USER\Software\Microsoft\Office\Outlook\Options\Mail This setting is tied to the following registry data, so this setting can also be configured by an administrator through a modification of the registry. :::image type="content" source="media/email-stays-in-outbox-until-manually-send-or-receive/send-immediately-when-connected-setting.png" alt-text="Screenshot of the Outlook Options window, where Send immediately when connected option is highlighted in Send and receive area." border="false"::: This problem can occur if the Send immediately when connected option is not enabled, as shown in the following figure from Outlook 2016. When this problem occurs, the message remains in your Outbox folder until you manually initiate a Send/Receive operation (for example, press F9 or select send or receive). :::image type="content" source="media/email-stays-in-outbox-until-manually-send-or-receive/outbox-folder-in-outlook.png" alt-text="Screenshot shows an email message stuck in the Outbox folder." border="false"::: When you send an e-mail message in Microsoft Outlook, the message may remain in your Outbox folder as shown in the following figure.
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Email stays in the Outbox folder until you manually initiate a send/receive operation in Outlook
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