Email certificate error on Passive server only

[giwm_e]

We're in an active/passive setup and recently upgraded from 16.0.5 to 16.0.6. Emails no longer send from our passive server with the following error:

An error occurred while attempting to establish an SSL or TLS connection. The host name did not match the name given in the server's SSL certificate.

I couldn't find any further information in a log file, but I may just not know where to look. Email works just fine on our Active server, so I can only assume it's an issue with the cert on the Passive server. However, I'm not sure why that cert would be used for communication with my email server.

Thoughts?