After the upgrade to 1.5.6 I checked the Email Bounce Log. I found two entries. Somehow they don't look like the classic bounced - I mean, it looks like someone have send (or tried to do) Spam. There is an attachment, too. As I am a layman to this, I don't know how to handle it. Would it help to understand, if I post the content here or as an (.txt) attachment?
You don't really need to do anything. The log is anything sent to that account. The address could be picked up through harvesting of hacked email boxes, for example. If it's not a bounce, you can just disregard it.