Too many mboxes (max 65536, you have 71571)

Well yes, I have a lot of mboxes. I'm not quite sure where mairix is
getting the 71571 figure from, since my ~/mail contains only 19734
files and some of those are git repository stuff and not mboxes.
I tried deleting .mairix_database and mairix stopped complaining, so
perhaps it had some old cached info and its count was off.

I have stuff like daily mboxes for spam and for viruses, daily mboxes
for all mail before any filtering, etc. So it's not inconceivable I'll
really reach 65k mailboxes some day. Can this limit be increased to
something less likely to be reachable?