A suspicious sender is a sender who sends email to recipients who have not given them express permission to do so.
A suspicious sender (some services use the term "black hat sender") is a sender or company who sends email to recipients that didn't specifically request to receive emails from that sender. It is somebody that sends unsolicited emails to recipients that never requested them.
Not all suspicious activity is intentional though. Even senders who have their recipient's permission but send an excessive amount of email or who send at an alarming frequency may be flagged by users as “spam." These senders will be identified by the recipient's ISP, which may throttle the sender's email, send it to spam traps, or totally block the emails.
If your emails are not getting the clicks and opens you expect, you may want to re-evaluate your sending habits. Also, please take a look at our email deliverability tips for help in being a trusted sender.
For more information: