Enabling opt-out list as workflow trigger
When cleaning up contact database, it's suggested to frequently gather contacts who have hard bounced in the past and import them into HubSpot as an opt-out list. To not get billed for these contacts anymore, one can then delete these contacts from your account.
After this, it'd be best to prevent these bounced contacts from being imported again to the system (e.g. from external CRM). At the moment Hubspot does not have a way to tell during import if this is happening, so one would need a workaround. Potentially you could create a workflow to set something up; so that you add the deleted contacts' emails as a workflow trigger and if a new contact is created with any of those emails then a custom notification would be sent to a user. But this is not optimal if working on a global set up with thousands of deleted contacts.
Thus, Hubspot should enable opt-out lists as workflow triggers. According to Hubspot support, this is at the moment not possible, since deleting a contact and then recreating it, you will not get the Contact history, such as list memberships or previous property history; it will in effect be a new contact.