Sage 100

Working with a client on their e-mail from Sage 100 via Paperless Office. They use an internal Outlook Exchange Server. A handful of users CAN send e-mail fine. However a FEW e-mail users fail; when trying to send an e-mail. Can someone tell me what setting needs to be changed in either the Domain Controller setup of that user, or in the Outlook setup of that user?

Users need to be allowed to ""send as"" the email address that is set up in Paperless as the SMTP authentication email address. This is set up in Exchange.

Thanks @PhilMcIntosh, That is what I thought I had already told the IT guy at my client; and wanted to double check before telling him the same thing again.