We recently sent a campaign for a client and received about 900 hard bounces (an expected rate. It was a large receiver list). A week later, another campaign was sent to the same list. We had expected to see very few hard bounces as a result of the automatic suppression list, however we still received about new 400 hard bounces (the suppression list appeared to be fine on checking). Why else could we have seen 400 new hard bounces from one week to another?
Could it be a case that the second batch to hard bounce had been 'soft bouncing' on previous campaigns, and it eventually hard bounced. I'm not 100% sure how CM works but a lot of email broadcasting systems allow a soft bounce limit - and once a contact reaches this limit they then hard bounce. Only a suggestion.