Hello @MartaG8 ,
What is the frequency of your subscriber update notification? Additionally, what is the source of the contacts? If the contacts are also bouncing as non-existent, then it's a simple enough matter to delete and unsub them while viewing your bounce report.
Most likely, a bad actor came across one of your sign-up forms, and added the gibberish addresses until the reCaptcha or a similar protocol kicked in to push back against additional new sign-ups without human verification. If this is becoming an issue for your contact management / billing, you might want to temporarily turn off or remove the sign-up form that’s causing the problem. Then, you can re-enable / re-add it after some time has passed.
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