Why is an mh format INBOX called #mhinbox instead of just INBOX?
It’s a long story. In brief, the mh format driver is less functional than any of the other drivers. It turned out that there were some users (including high-level administrators) who tried mh years ago and no longer use it, but still had an mh profile left behind. When the mh driver used INBOX, it would see the mh profile, and proceed to move the user’s INBOX into the mh format INBOX. This caused considerable confusion as some things stopped working.