I started getting some spam and, when the first piece arrived, I used to the Apple Mail "Rules" function to put in a 'Delete if it Contains', but then it kept showing up and I kept writing new rules. This morning I reviewed the rules written so far and found that addresses differed slightly.

6857.ugKl1l36iRjSdt84elbv@i8shux.d63rrwu7t5.us
6857.ugKl1l36iRjSdt84elbv@gua84k.gng2y96knq.us
6857.ugKl1l36iRjSdt84elbv@rxb2b5.wp22k437kv.us
6857.ugKl1l36iRjSdt84elbv@67dnhr.mv6wysa1p1.us

Everything is common up to the @ so I wrote a new rule that looks only for addresses "Containing" 6857.ugKl1l36iRjSdt84elbv@ Hopefully that'll do it. And, hopefully this experience will help anyone else with a similar issue.