Bounced Email Woes

[HM666]

Having a severe bounce email problem with a client's site since the host "fixed" their hosting. They "fixed" it by moving them to a bigger more expensive hosting plan which is not working much better than the plan they had before. To make a long story short it never fixed the initial problem that we had to begin with and now the client gets hundreds of bounced emails pretty much every day since the switch.

The hosting company in their infinite wisdom cited that we should turn off the MAILER-DAEMON and that would solve the problem completely. If we just turned this off all our troubles would be over. I'm not so sure about that personally. I think they are idiots personally. This is the information they gave my client on turning it off:

This file needs to be edited via ssh command line not via WHM/CPANEL or FTP.

You would add/edit the entry in the /etc/aliases file on the server to read: -

MAILER-DAEMON: /dev/null

This was the extent of the instructions they gave. I really feel this is just a band aid to a much larger server problem they have. When my client asked them if they would edit it they said they would "do it this time but charge him if they had to do it again". The bounced emails stopped for a couple of weeks and now they are back, when my client emailed them again they said the same thing as above to edit the file. But didn't they do that already???

So my question to everyone is how do I go about trying to find and fix this problem with the email? How do we find why there are hundreds of bounced emails in this account. What steps should I take? Is it safe to edit this file via SSH? Or is the host full of s***? If its ok to edit this file via SSH? How do I navigate to it using SSH? Its not that common that I use SSH to design a web site, so I do not know the commands to navigate to files or where they are. And of course there was nothing on the host's site that was remotely helpful.

What I have tried already:

1. Checking the settings in the WHM.
2. Checking the settings in the cPanel.
3. Enabled SpamAssassin.
4. Removed email accounts that seemed wrong that the client did not remember creating.
5. Have done several hours of research online/google looking for an answer and have found nothing that tells me what I need so far.

Any help would be great! Thanks.