How reputation and content interact

Recently, one of my clients had a new employee make a mistake and ended up sending newsletters to people in their database that had not subscribed to those particular newsletters. This resulted in their recipients getting 3 extra emails from them. These things happen, people fat-finger database queries or aren’t as careful with segmentation as they should be.
My clients were predictably unhappy about sending mail their users hadn’t signed up for and asked me what to do to fix their reputation. I advised they not do anything other than make sure they don’t do that again. The first send after their screw-up had their standard 100% inbox delivery. The second send had a significant problem with bulk foldering at Hotmail and Yahoo. The third send had their standard 100% inbox delivery.
So what happened on the second send? It appears that on that send they had a link or other content that “filled the bucket.” Generally, their IP reputation is high enough that content isn’t sufficient to send their mail into the bulk folder. However, their reputation dipped based on the mistake last week, and thus the marginal content caused the bulk foldering.
Overall, these are senders with a good reputation. Their screw up wasn’t enough to damage their delivery itself, but may have contributed to all their mail going into the bulk folder the other day. I expect that their reputation will rebound quickly and they will be able to send the same content they did and see it in the inbox.