Auth problems to SMTP relay

The last time I was a Comcast Business customer, they gave me a default email address at comcastbiz.net associated with the account.  That was my Comcast portal login, and I was able to use that email and password to send mail through the SMTP relay.

Now I'm a Comcast customer again, and this time they did not give me a default comcastbiz.net address.  Instead, I log into the Business portal using a non-Comcast email address as my user id.  That works for the portal, but not for the relay.  No matter what combination of credentials and port I use, invariably the response is 535 5.7.0 Reject: Not our customer, or some close variation of that.

I've called support but nobody who picks up the phone even knows what an SMTP relay is.  It's hard to get them to understand what relay credentials are when I first have to explain what a relay does and why I want it.  One of them suggested I might set up my free domain at the portal, so I did that and it did give me an email address at comcastbiz.net, but that email address doesn't work any better for this.

Surely there must be SOMEONE at support who knows what a relay is and how to authenticate to it.  I remember their support being better than this the last time.