Mailfile mode 200

When we expire an account here (due to nonpayment), we change the mode on
their mail file to 200. Which means they can still receive mail, but not
read it. How can exim's "appendfile" director be adjusted to handle this?

I don't want exim to change the mode of the file to 200, but 200 is also
narrower than the desired mode of 600, so the message doesn't get
delivered.