Not on SM16, but we started having this issue with Clam after upgrading to 15.7. We fixed the issue by adjusting some Clam and Spool values. Those settings still exist in SM16 so I imagine it will work for you too.
See my comments in this thread for details, https://portal.smartertools.com/community/a2881/issues-with-clamav-managing-it-by-smartermail-and-issues-with-clamav.aspx.
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