How to allow ‘outside email filing’ in Exchange server

Hello,
We are testing the new Exchange Server Integration and we have a dilemma. There are users "outside" our organization with no exchange account who at present need to file emails and without synchronization – only filing emails – it looks impossible. If a user without an Exchange account initiates filing an email we get an error message as:
This action cannot be performed as you are not enabled on CRM server to synchronize with Exchange. Please contact your CRM system administrator.
I used the option on v7.1c with the CRM Client Application plug-in version 7.1.0.3. Is there a way to enable non-exchange users to file emails without creating extra exchange accounts?
My target is to send an email with the help of the .net API, and have known much of the procedure there using Message block; anyhow I am facing the last obstacle of my message sending failure. I may also be able to use Add Content my Message block. Execute; and that will show my message email with a manual "Send" button, although I in fact intend for it completely in the setting.
I doubt I require the use of anything in terms of my Message block. Execute (arg); but as the documents suggest about this function, with the suitable case it will cause the message block to perform, I am not sure exactly what I require in this case.
Will somebody tell me the solution please?
Thank you so much.
