Setting up a VPN is the hard way. You could achieve the same solution by just running your SMTP server on a different port to accept your connections from your client.
We have a smtp server that listens on 25, 465, 587 and 12196 - so that you'll be very unlucky indeed if you can never reach it, iresepctive of where you travel or ISP/Connectivity method you use.
BTW this also uses SMTPS and Authentication (via starttls (25,12196,587 or wrapped port 465))