[Exim] maildir_use_size_file, quota_size_regex, maildir_tag

If maildir_use_size_file is set True, are the maildir_tag and
quota_size_regex options necessary? If no, does having them all
enabled offer a theoretical performance improvement?

I am debugging a problem where IMAP reports a different size than du(1)
reports (excluding the same files, of course).

I am using Courier IMAP with Exim 4.33. The maildir file is as
follows:

    20971520S,0C
    15438488 3179
    909 1

Exim is configured thus:

    maildir_quota_directory_regex = ^(?:cur|new|\.(?!Trash).*)$

From what I have read of the documentation for Courier, it tallies all
maildir directories except Trash. I'm hoping this is consistent with
the regex in Exim.

To verify with du, I run:

    # du -b --max-depth=1 --exclude='spambayes.db' --exclude='.Trash'
    21687808        ./cur
    512     ./new
    512     ./tmp
    6144    ./.Sent
    512     ./courierimapkeywords
    6144    ./.Drafts
    21894656        .

This account is pretty much a catch basin for spam, but not
advertised as a honey-pot. Before running these tests I removed the
maildir file and sent a small test email so that it would be
recreated.

I have also noticed that some of the mail files under ./new do not
have a S= suffix. My gut-feeling is this is the cause of observed
inconsistency.

Thank you all very much for reading this. Any suggestions would be
wonderful.

take care,
hank