Yesterday I opened my mail client thunderbird and suddenly a message pops up “Sending password did not suceed”. Wow, great!

After doing some research I finally found a solution. And it’s a well known bug:

The reason this happens is because many cPanel installs were done on top of a Linux install that had a POP3 daemon run from xinetd. You will need to change into the /etc/xinetd.d directory from ssh and find the popa3d and popa3ds files. The files look like this:

# default: off
# description: The POP3 service allows remote users to access their mail \
#              using an POP3 client such as Netscape Communicator, mutt, \
#              or fetchmail.
service pop3
{
        disable = no
        socket_type             = stream
        wait                    = no
        user                    = root
        server                  = /usr/sbin/popa3d
}
 
What you need to do is set the 'disable' parameter to 'yes'. It should look like this when you are done:
 
# default: off
# description: The POP3 service allows remote users to access their mail \
#              using an POP3 client such as Netscape Communicator, mutt, \
#              or fetchmail.
service pop3
{
        disable = yes
        socket_type             = stream
        wait                    = no
        user                    = root
        server                  = /usr/sbin/popa3d
}

Repeat the same for popa3ds and then run:

service xinetd stop
service cpanel restart
service xinetd start

Source

After that simply restart “POP3 service” via WHM/Cpanel and you’ll be fine again.
Annoying, but now it’s fixed!