When you try to send emails from your software using Outlook, you can see the error:

'Unable to send all emails. Error with the address. Unknown Error. Check Windows event Log for more details.'

When you check your Windows Event Log, you'll see:

'System.Invalid.CastException: Unable to case COM object of type

'Microsoft.Office.Interop.Outlook.ApplicationClass' to interface type 'Microsoft.Office.Interop.Outlook.Application'. This operation failed because the QueryInterface call on the COM component for the interface with IID ... failed due to the following error: Library not registered...' 

This error occurs when you need to register your Microsoft Outlook installation.

Repair Microsoft Office installation

1. To resolve this issue, run a repair installation of Microsoft Office.
2. If the issue persists, uninstall then reinstall Microsoft Office. For help with doing this, contact your IT support.

Issue persists

If the issue still occurs, there's an issue with the Microsoft Office entries in your Microsoft registry. Your IT support need to troubleshoot your Outlook installation to resolve this.