Mình đang cần làm một mô hình Email như sau:

Hosting Email ------ POP3-------- Exchange --------> Local Users

Vấn để đang nằm ngay POP3.

Mình không biết có software nào chạy trên Linux (CentOS) để làm việc này như Mdeamon hay POPbeamer trên Windows!!!

Thân. 

không thấy ai nhắc tới exim cả ?
mà không biết mail client nào trên Linux tốt 

Exim is a message transfer agent (MTA) developed at the University of Cambridge for use on Unix systems connected to the Internet. It is freely available under the terms of the GNU General Public Licence. In style it is similar to Smail 3, but its facilities are more general. There is a great deal of flexibility in the way mail can be routed, and there are extensive facilities for checking incoming mail. Exim can be installed in place of Sendmail, although the configuration of Exim is quite different. 

vậy dovecot đóng vai trò gì 

Dovecot is an open source IMAP and POP3 email server for Linux/UNIX-like systems, written with security primarily in mind. Dovecot is an excellent choice for both small and large installations. It's fast, simple to set up, requires no special administration and it uses very little memory.