email problem: status=deferred (maildir delivery failed: create maildir file /home/vmail/

try first, ehcp upgrade to latest version,

open logs,
tail -f /var/log/ehcp.log
tail -f /var/log/syslog

ehcp gui->options -> fix mail config

if problem persists,

groupdel vmail
userdel vmail
echo "----------- Other user/group with uid/gid of 5000, you need to delete them, if any -----------";
grep 5000 /etc/passwd
grep 5000 /etc/group
echo "----------- ----------- ----------- ----------- ----------- ----------- ----------- ----------";
groupadd -g 5000 vmail
useradd -g vmail -u 5000 vmail -d /home/vmail -m
chown -Rf vmail /home/vmail
adduser postfix sasl

if still persists,
make sure you installed onto a clean ubuntu system,
check if /etc/passwd contains other user with uid 5000, group with gid 5000