You could create a local alias on your SME for the reports that forwards email to the ultimate destination, then reconfigure the external win computer to email the reports to the local alias on the SME.
Or, you could use "plink" in a batch file on the windows box to open an SSH tunnel to the SME before sending the email, then configure the windows box to use itself for the mail server (you'd need to configure a key pair to allow password-less login).
Or, you could VPN from the win box to the SME (I once knew how to open a VPN from a batch file)
Or, You could modify your SME to allow open relay from the IP address of the external win computer using "AllowHosts"
Or, you could install an SMTP server near the win box that knows how to do outbound SMTP authentication (the SMTP server that comes with all Windows server OS's will do this, I think).