pop3 server on linux system

Hello,

I have 2 clients in my webhosting business who need to send a lot of mails.
One has a live helpdesk trough mailing, and one has a quite large mailing list.
The problem is that the current mail server tends to fall down when lots of users are mailing (especially smtp).

I have bought a dedicated system for the mail purpose only.
What do I need to install, what do I need to configure, and so on?
I connect to it using bash cause it's a raw install.


thanks in advance

Rémisan