[exim] Question about umasks and exim

I am running exim4 and dovecot and I am having some issues with the file
permissions on the emails that are being being moved once the email has
been delivered. I am using the maildir format mailbox and accessing all
the email through IMAP only.

When the mail is originally delivered the permissions are set correctly,
but then if the user moves the message from the Inbox to a different
folder, the permissions are then set to only rw by user and nothing for
the group or world.

My question is both exim4 and dovecot have locations to set the default
umask on files and directories. So which application has control of the
umask on files that are moved? I thought it might be dovecot, but I have
played around with that particular mask and nothing seems to be changing.

Thanks,
-Eric