So your cert should currently have the chain DST Root CA X3 > R3 > Your cert.
When you export the cert to use it with hmailserver how do you do that?
When you verified the cert with the digicert tool, did you use the original PFX that Certify generates or did you use your exported/converted cert that you use for hmailserver?
Here is an example of how another user exports to hmailserver: Hmailserver and new version 5 CTW
It’s not impossible that we have a bug but if we do it would be more likely to be related to a specific export as these are less commonly used than the core cert renewal functions.