Yes. The D3 Email Gateway is not a mail server. It interfaces with existing mail servers.
Your D3 system uses our Email Gateway to send an email. Our TCL command SENDREC will take the D3 record and hand it off to your mail server.
You also must make sure your mail server trusts the IP Address of your D3 system. If you cannot do this then you must provide the SENDREC command with a userid and password to authenticate with on your mail server.