When you send out a Haulix email invitation, we track the email's journey to the contact. You'll know when and if it was Delivered, Opened, Bounced, Dropped or if the contact Unsubscribed.
All of that email tracking information can be managed from your Invitation Activity screen.
An email can Bounce for many reasons. Perhaps the contact's mailbox is full, or their email address is invalid. If an email does Bounce or the contact Unsubscribes, Haulix will obey those results and not send to that contact anymore.
It's important that you keep your contact list clean. Periodically look through your Bounce list. If there are invalid email address-related bounces or drops, then remove that contact from your list.
If this was an Unsubscribe or a Bounce and you would like to give the contact another chance to receive invitations, then you would choose the "delete from _____ list" option.