Recently we have been getting Undelivered Mail bounce notification when sending invoices.

Reason: uncategorized-bounce 

Some go through no problem some bounce back.  We recently sent 10 invoices, 6 received bounce notifications. After reaching out to the customer only 5 weren't received of the 6 bounces.

Some I am not really sure how to resolve this and how to know which are real bounces and which are not.