Do you get any Ribbon UI errors?
By default, if an VSTO Add-in attempts to manipulate the Microsoft Office user interface (UI) and fails, no error message is displayed. However, you can configure Microsoft Office applications to display messages for errors that relate to the UI. You can use these messages to help determine why a custom Ribbon does not appear, or why a Ribbon appears but no controls appear.
To show VSTO Add-in user interface errors
1. Start the application.
2. Click the File tab.
3. Click Options.
4. In the categories pane, click Advanced.
5. In the details pane, select Show VSTO Add-in user interface errors, and then click OK.
For Outlook, the Show VSTO Add-in user interface errors checkbox is located in the Developer section of the details pane. For other applications, the checkbox is located in the General section of the details pane.
Most probably the TabMail id doesn't exist in all contexts and you get an UI error. So, a custom UI is not shown finally. Am I right?