You said before have a problem with vpopmail-toaster.
Vpopmail just interface to login process. Pop3 and Imap handle by courier. Smtp handle by qmail.
So, your issue related to courier instead vpopmail or qmail.
So try increasing SOFTLIMITVAR inside /var/qmail/supervise/pop3/run and /var/qmail/supervise/pop3-ssl/run to:
if [ `uname -m` = 'x86_64' ] ; then
SOFTLIMITVAR="64000000"
else
SOFTLIMITVAR="9000000"
fi
Many reference advice increase softlimit from 9000000 to 48000000 for centos 64bit but this advice relate to centos 5. Maybe Centos 6 need more.
That it