Comment 2 for bug 52899

There's a good summary of this problem here:
http://homepages.tesco.net/~J.deBoynePollard/FGA/mail-mbox-formats.html

There are several different, and unfortunately incompatible, mbox formats. Some of them escape "From" and some of them don't. You also can't reliably tell which you've got, and mutt seems to make use of the Content-length header if it's there. There's a lengthy discussion of this at http://dev.mutt.org/trac/ticket/714

Unless this has been causing trouble, I wouldn't worry too much about it. And if it is, I'd switch to Maildir entirely! The mutt bug has been open for 7 years, and the discussion is still going on!