When you use a MAPI-compliant program to create a new email message, the message may not contain the default Microsoft Outlook email signature. This includes, but is not limited to, the following scenarios:
You use Send As or other Send commands from another Microsoft Office application, such as Microsoft Word, Microsoft Excel, or Microsoft PowerPoint. This scenario typically occurs when you use a command to send the current file as an attachment.
You use Email or Send to Mail Recipient in Windows Explorer.
You use the SendObject method or command from Microsoft Access, and you use the EditMessage parameter to display the message instead of sending it immediately.
A custom solution programmatically creates a new email message window by using Simple MAPI or Extended MAPI.
Any of these scenarios may use Simple MAPI to generate the email message. In this case, while Outlook is handling the MAPI calls and generating the message, Outlook does not handle this scenario in the same manner as it does if you create a new message directly in Outlook.
Use the Signature button on the Message tab of the Ribbon to insert the signature manually.
Note There are no plans to change this behavior in Microsoft Outlook 2013 or in earlier versions of Outlook.
Generally, it is easy to tell if a new message was created by using Simple MAPI. In these cases, the window is modal, and this means that the focus cannot be taken away from the email message window until the window is closed, or the message is sent.