I have just installed my new BT-445C and I am occasionally getting
   this message.

   charon /netbsd: bt0: mbi at 0xf86779e4 should be found, stat=81..resync

   A missing mailbox--is it serious?

Technically, what that means is that the card generated a `mail box in
full' interrupt, but the driver couldn't find any filled mailboxes to
process.  The net effect of this is that the interrupt is ignored.

If there really is a full mailbox, this will result in the card
sending another interrupt (eventually; perhaps as late as the next
time there is a mailbox put on the inbound queue).

This is only `serious' if the card is generating an infinite stream of
interrupts.  From what you said, I gather that this isn't the case.