Today I tried grepping for some stuff in a large Maildir and received the aforementioned error.
A nice workaround is to use:
grep -r "blah blah" /path/to/dirinstead of
grep "blah blah" /path/to/dir/*
HTH!
grep -r "blah blah" /path/to/dirinstead of
grep "blah blah" /path/to/dir/*