When you send an e-mail message, the message may remain in your Outbox folder as shown in the following figure.
When this problem occurs, the message remains in your Outbox folder until you manually initiate a Send/Receive operation (for example, press F9 or click Send/Receive).
Warning Serious problems might occur if you modify the registry incorrectly by using Registry Editor or by using another method. These problems might require that you reinstall the operating system. Microsoft cannot guarantee that these problems can be solved. Modify the registry at your own risk.
This problem can occur if the Send immediately when connected option is not enabled, as shown in the following figure from Outlook 2010.
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.
Note In the above registry key paths, x.0 represents the Outlook version (15.0 = Outlook 2013, 14.0 = Outlook 2010, 12.0 = Outlook 2007, 11.0 = Outlook 2003).
Please use the following steps to re-enable the Send immediately when connected option.
Outlook 2013 or Outlook 2010
On the File tab, click Options.
In the Outlook Options dialog box, click Advanced.
In the Send and receive section, click to enable Send immediately when connected.
Outlook 2007 and Outlook 2003
On the Tools menu, click Options.
In the Options dialog box, select the Mail Setup tab.
In the Send/Receive section, click to enable Send immediately when connected.
Note: If you are unable to re-enable this setting because it is grayed out, then the setting is being managed by group policy. In this situation, please contact your administrator to have this group policy removed.