Error when emailing an invoice as attachment

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Hi,

One of the computers in our company is giving the following error when emailing an invoice (this happens for other documents on that computer as well):

Error with the address ****@*****.com : Unknown Error. Please check the Windows Event Log for more details.

The invoice file is stored on a network drive so we all use it and it works fine on other computers. Normally it creates a draft in Outlook with the invoice as an attachment. 

What causes this local error?