If VPOP3 will be sending mail using SMTP Direct sending (MX sending), then all you need to tell VPOP3 is which DNS servers it should use to lookup MX records.
In the DNS Server(s) to use box, type in the IP addresses of the DNS servers which VPOP3 should use. If you want VPOP3 to try multiple DNS servers, then separate them with semi-colon characters (;). If you leave this box blank, then VPOP3 will use the DNS servers defined in the Misc Settings (or used by Windows, if no DNS servers are set there).
Press Finish to create the new Connection and Mail Sender.