At Zoho Campaigns, we store hard bounce contacts in a separate mailing list. Whenever our users try to add new contacts to their mailing list, we will verify it with the hard bounce list. If the user tries to add contacts that are already in the hard bounce list, the system will ignore those contacts.

 

Sometimes, if our users try sending email to an active contact and the contact's mailbox is full, the email will bounce and it will be categorized as a soft bounce. After seven consecutive soft bounces, the contact will be added to the hard bounce list and your future campaigns will not be sent to those contacts.