Cannot delete or junk messages with malware attachments in Mail

Since installing Sierra (and thus Mail 10.0) I am unable to delete, junk or even move messages with malware attachments.

I have on-access scanning enabled for Adware and PUA's.  The action set for When a threat is found is Deny access.

Prior to installing Sierra, for messages with malware attachments I was able either to Delete or Move to Junk Mail, and have been doing so for many years — and have made no change to the above settings.  Now either of these actions results in a message about a problem with access privileges.  However, the file privileges are fine.  The Sophos log shows that attempts to delete messages with malware attachments are being blocked by Sophos.

One can avoid this problem by temporarily turning off on-access scanning, deleting the problem messages, and restoring on-access scanning.  But this is a pain.  And using the Quarantine Manager is worse — it takes a long time to effect Clean Up This Threat at the best of times, and now seems to be broken as well (it says it can clean up a threat, tries, and then indicates that it needs to

Is this a bug in Sophos Anti-Virus for Mac Home Edition?  Is this something that was broken by Sierra?  Should I be using other settings to be able to delete or move email manually?  Or is this some other kind of problem?

Thanks in advance ...