There’s a bug in cPanel which prevents user from checking mail from mail client, while you can login to webmail without a problem and you can send out email without a problem.

To fix this, SSH into server/vps, try to receive email within mail client using account that client provided and then run: tail -f /var/log/messages

just to make sure it’s this problem. It should be something similar to this:

server1 popa3d(pam_unix)[13128]: check pass; user unknown
server1 popa3d(pam_unix)[13128]: authentication failure; logname= uid=0 euid=0 tty= ruser= rhost=
server1 popa3d[13128]: Authentication failed for UNKNOWN USER from 127.0.0.1

then: cd /etc/xinetd.d/ and edit these files:

pop3s
popa3d
popa3ds
cpimap

You’ll see one line saying:

disable = ‘no’

make it

disable = ‘yes’

Save and exit. Then restart xinetd and cppop and it should be alright.