Hi @FSandbox,
Thanks for reaching out to the Community!
This is usually displayed when an email attempt is made for a contact who has previously bounced.
I’d like to share this article which explains all the possible email errors, I’d recommend taking a look 
I also wanted to share this knowledge base article where you can find all the email bounce types.
Can you take a look through these articles please and tell us if you’re seeing a bounce error when you send an email to the contact from your email inbox outside of HubSpot? Thank you!
Cheers,
Kristen