You could try changing the Default Domain to that from which you are sending. Ie, if you email address was
bob@wingate.com, then open Gatekeeper, under the System tab, go to Email properties, and duble click on your default domain (which will be your computer name). Change this to wingate.com, and click OK. You will get a couple of notification boxes, informing you of the items that are being automatically updated. You shoudl now be able to sned as per normal.
The other way would be to create an SMTP proxy under the Services tab in Gatekeeper (by right clicking in said tab and selecting 'New Service - SMTP mapping); however i would recommend the first way, as this gives you additional control over your email, as well as antivirus protection (if you have AV for WinGate).
Regards
Neil