Suspicious Sender
Reputation. An email sender who sends email messages to recipients that never granted explicit permission to that sender.
Some suspicious activity could be unintentional. If a trusted sender send either an excessive amount of email or who send at an alarming frequency, recipients might flag their messages as spam. An recipient ISP might throttle the sender's email messages, send them to spam traps, or block the emails outright.
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Suspicious sender, not black hat sender
You might encounter the term black hat sender. The term suspicious sender replaces that term and eliminates any potential racial or cultural connotations associated with the earlier term.