Newly arrived mail is marked as "read"

I'm in the process of testing the upgrade from a CentOS 6 box running 
2.3.16 to a CentOS 8 box running 3.0.7.

The upgrade has been less troublesome than expected, but I'm seeing a 
strange problem with newly received mail being marked as "read" even 
though they have not been opened by the client.  This is the case for 
both mail sent locally as well as mail received externally (Gmail).

Can anyone point me in the right direction on where to look to fix this?

Thanks.
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