You can still send a one-to-one email to a contact imported through an opt-out list as long as you provide one time legal basis with each one-to-one send.
Prepare your opt-out list
In Excel, add an Email column with the email addresses of the contacts to be opted out from receiving emails. Save the file in a CSV format.
Import your opt-out list
In your HubSpot account, navigate to Contacts > Contacts.
In the upper right, click Import.
Click Start an import.
Select An opt-out list, then click Next.
Click Select a file. In the dialog box, select the import file from your device.
Click the Email field column header dropdown menu and select the column header of the column in your import file that has the Email field.
Click Next, confirm your choice.
In the text field, enter the number of contacts to confirm they will be opted out from receiving emails.
In the bottom right, click Begin import.
After your opt-out list has been imported and processed, the Unsubscribed from all email property of all the list's contacts will be set to true.
Will these contacts count to your billable contact total?
Contacts who are imported on an opt-out list will not be added to your billable contact total because it does not create contacts in your HubSpot database. Instead, it marks the email addresses on the list as ineligible to receive emails from your account. These email addresses will be included if you ever export a list of bounced and unsubscribed contacts.
However, if there are email addresses in your opt-out import that currently exist as HubSpot contacts, while they will be automatically unsubscribed, they still exist as contacts and will count towards your billable contact total. To prevent these contacts from being included in your billable total, you can delete them and then re-import as an opt-out list. They will not exist as contacts in your portal, and only their email addresses will be marked as ineligible.