[Exim] Lines: xxx mail header and maildir

So, I recently switched to maildir delivery with the following
transport:

# Those options are for Maildir+ (Courier Imap), but compatible with regular
# Maildir
local_delivery_maildir:
driver = appendfile
maildir_format
create_directory
maildir_tag = ,S=$message_size
directory = /home/${local_part}/Maildir/
return_path_add
delivery_date_add
envelope_to_add
group = mail
mode = 0660
no_mode_fail_narrower
use_fcntl_lock
use_lockfile


The problem is that I then read my mail with mutt, which reads the
Lines: header to show me how many lines are in the message it tells me 0
for all the maildir messages since they lack that header.
Apparently the header doesn't get added anywhere.

Some of my mail also goes to procmail, which also delivers to maildir,
and that doesn't add a Lines: header either.

Ok, I'm curious now: who is responsible for adding that Lines header?
Exim?
The MDA?
Other?

Thanks
Marc
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