In the Helm
config tool, make sure the SMTP settings are correct (the server should
be your machine name, and the port should be 25).

As per the
MailEnable documentation, ensure that the mail relays are correct. Go
to MailEnable and right click the SMTP service and choose Properties.
Click the Relay tab. Ensure that the only box ticked is the “Allow
relay for privileged IP Ranges” box.

Click the Privileged IP
Ranges button, and in there enter all the IPs your machine uses (go to
a DOS prompt and type ipconfig /all to retrieve these).