Short answer
HubSpot's KB: contacts with suspicious email or form activity, such as repeated spam complaints or bounces, may be quarantined and dropped from sends. View them in the Quarantined Contacts segment. They can't be manually removed; they must re-engage.
1. In HubSpot
The relevant screen in my demo portal.
2. Worth knowing
Check form spam protection if many appear at once.
3. Related
Form spam and bounced contacts.