New problem solver
"Not Trusted" certificate presented when adding a Comcast Business Exchange account to iOS
A certificate issued by Comodo, expiring in 2016, is presented when adding a Comcast Business email address as an Exchange account.
iOS 9.2 fails to trust this certificate by default. (Missing intermediate CA? Missing domain name? *shrug* iOS didn't provide detail on why it was Not Trusted)
Besides that, the certificate has a SHA-1 signature. (Please stop using SHA-1 for signatures. Everybody is migrating away from this weak hashing algorithm --- NIST, Microsoft, Google.)
Workaround: I choose to NOT trust this certificate.
Next, I was presented with a dialog to fill in the server name (use the DNS Settings in the Business account site) and make the account name be the same as the full email address.
I did that and the account was added.
Hey Comcast, the "Not Trusted" certificate warning is a problem you need to fix.