This is something every sysadmin knows, or shoul know, but maybe Desktop Linux users does not know.
Yes it is not too useful for Desktop users anyway but, in case you do not know and might need to enable or disable the loggin ability of some users here is how to.
First let’s see why we would like to do such a thing (blocking a user from loggin).
If you have a Linux System that works as email server, you do not need to give loggin permissions to all the users of the box, specially if you allow them to change their passwords using poppassd, as the normal user usually use weak passwords, and if this user has full access to the server, someone may gess his password and hack your Linux.
So for the users you want to be able to loggin in to the server with ssh or from the console use a line like this in the password file /etc/passwd
And for the ones you do not want to have access to the server use:
Usually you only need to change the /bin/bash to /bin/false once the user is created by editing (as root) the /etc/passwd file.
That user will still be able to use the pop3 / imap / smtp.