One bug is to do with insufficient checking of a counter, so it wraps at 2GB. The worst case here is that the program crashes, I believe. I don't think it is exploitable.
Another bug is a crash on parsing a slightly weird
Then there are various other things that you could consider bugs. qmail doesn't comply with the current Internet standards. If you restrict bugs to mean "doesn't function as originally designed", then these are not bugs. But I think they would be considered bugs by most people, since they affect qmail's interaction with other mail software.