After upgrading to 8860 a week ago, I noticed a huge jump in emails in my Spool for extended periods. I finally figured out what is going on today. In my Delivery logs for the failed recipients, I'm seeing this:
WARNING: Certificate name mismatch. Expected Hostname: smtp4.[DOMAIN].com, Certificate Information: Subject=CN=spamav2.[DOMAIN].com Issuer=CN=R3, O=Let's Encrypt, C=US
This has occurred for 3 different clients sending to 5 different domains. In order to get the emails to deliver, I have to turn off both "Enable TLS if supported by the remote server" and "Force all traffic over HTTPS". I did not have to disable those prior to this build.
Owner of Efficion Consulting