HubSpot will quarantine contacts from an imported list or after a marketing email send if any of the following conditions are met:
The list directly contributes to a high hard bounce rate (15% or higher) on multiple emails sent through HubSpot. For reference, a hard bounce rate of 1% is typically considered normal in the email industry.
The list contains email addresses with a known history of high hard bounce rates based on HubSpot's historical email data. These emails are also referred to as global bounces.
Contacts were added through suspicious form activity. This scenario is most likely caused by a third-party with the intent of spamming the recipients' inboxes.
How quarantined contacts and lists work
After contacts on a list have been quarantined, you won't be able to send marketing emails, including automated workflow emails, to those contacts. If those quarantined contacts are also present in other lists, they'll be dropped when sending to those lists.
The following contacts will be exempt from the quarantine:
Contacts who opened or clicked one of your marketing emails.
Any contacts who opted in through a valid form submission.