This may be caused if the Address Processing Options in Acknowledgements are set to send mail to the contacts of an organization.

Here are steps to check your Address Processing Options:
  1. Go to Marketing & Communications > Acknowledgements.
  2. Click Edit next to Processing options.
  3. Click on the pencil icon next to Address Processing Options.
  4. Go to the Organization Tab.
  5. If you have the option marked to mail to contacts at the organization, you need to check the address on your contacts.
Here are the steps to check the address listed on the organization contact:
  1. Go to the organization's constituent record.
  2. Go to the Relationships tab.
  3. Click on the primary contact (or contact type listed in the Address Processing Options), and click Edit.
  4. Go to the Contact information tab.
  5. See if the address here is the outdated address appearing in the results of Acknowledgements.
  6. If so, click Edit under contact address, and enter in the new address.
  7. Click Ok.