OS X Server presents wrong certificate

Greetings. I've just set up OS X server to host shared (non-iCloud) calendars, reminders, and contacts. We have a wildcard certificate for our domain purchased from a certificate authority. I have installed that authority's cert in Keychain Access and pointed to the wildcard cert and key under "Certificates." This is the only cert that OS X Server uses.

Sharing works OK, but there's problem with push notifications via an external mail server. Via packet captures and logs, it seems OS X server still presents "hostname.local" instead of the wildcard cert during the SSL/TLS handshake. If I disable SSL for push notifications (both send and receive), notifications work OK. So, I think the wrong cert is the problem here.

How to get OS X Server to present the wildcard cert for both SMTP and IMAPS connections?

If it matters, under System Preferences/Sharing, this system only has "hostname.local" set. I have not set up a FQDN and do not run a DNS server on this machine. If an FQDN is needed here, please advise on how to set it up.

Thanks!