fixing bounce processing and extracting message fixing from Mail::Message

Ok, this is a rather big change and I tried to split it up as good as possible into different commits, all representing the most atomic kind of change possible.

All the commit messages should contain explanations about the reasoning and the solution itself. Also I tried to improve code documentation where possible. AND imho I also improved test coverage, as well as kept all the existing tests as much as possible.

This request is here for discussion, though I really tried to come up with other possible solutions and this was the least invasive as possible, while still improving the overall situation and acutally fixing issue #234 (closed)