I just updated my SSL certificate for another year and installed it on the server. If you go directly to the webmail interface, the certificate there shows the new one, not the old expired one.
However, when I opened up Outlook 2016 after the SLL certificate was changed, all of my accounts gave errors that the certificate was expired. When I viewed the certificates shown in Outlook, sure enough they were all still showing the old expired certificate.
Any idea why this might be happening?