Relay between SMTP IIS virtual hosts in the same server

Question

Hi,

My scripts send mail to a virtual SMTP from an IIS server configured with an internal FQDN, and that cannot be changed by now.

So in order to Relay to Office with TLS activated I need another Virtual SMTP with a proper PKI FQDN.

I have this working just fine with 2 Virtual SMTP in the samehost, with two IPs on the server.

So the "internal SMTP" relay to the "External SMTP" and from there to Office all the way with TLS.

Is there anyway to set-up the same thing with only one IP?

All the tests I've done with a sinlge IP end up in a 5.3.5 loop error (changing FQDNs, SmartHost, Try direct delivery first, ..)

Or even better, is there better way to set up this?

I understand I'm not the only one that need to keep the old good (bad) internal FQDNs to get the email and at the same time realying to Office with full TLS., whie keeping resources (new servers, new IPs, etc) at the lowest possible

Thank you