User's email is in a User-defined list and opted out of the email. This put a do not contact flag on the constituents record in CRM. If you remove the do not contact flag from CRM, the user defined list is still globally opting the email out of emails, with no way to opt the user back in. Once an email uses the user-defined list there is not a way to delete the list.
We are currently evaluating this issue for a fix in a future service pack.
Steps to Duplicate
1. Create a user-defined list in BBIS 2. Enter in name & email 3. Send an email using user-defined list 4. Opt out of the email that was sent 5. In CRM, see that record has do not contact flag 6. Delete the do not contact flag 7. Send a Direct marketing email out of CRM to email 8. CRM/BBIS will not send the email as it has opted out in the user-defined list