Create Missing tmp maildir folders
From James Dooley's Wiki
Overview
If a customer has an issue where mail is not showing up in their mailboxes but it does look like the server has the mail, and mailperms has not solved the problem, it may be due to the tmp directory being missing.
From the users mail directory you can run this script to generate tmp directories. Keep in mind that right now it is only set for a single user and will only fix sub directories. You can manually create tmp folders also using:
mkdir tmp
chown $user. tmp
chmod 700 tmp
Script
for dir in $(find -type d -maxdepth 1 | grep "\./\."); do mkdir $dir/tmp; chown $user. $dir/tmp; chmod 700 $dir/tmp;done
What to change
You will need to change $user to the actual account owner.
Also this script must be run in /home/$user/mail/.user@domain.com/