just a quick update...........
I have 2 servers on two different broadband ISP's
server 1 has been running for 5 years plus on debian/ispconfig2 with no real problems.
It can receive mail from server2 and from gmail and yahoo etc.
Server2 is the one that I am testing now and is also running on debian but with ispconfig3.
It can send and receive mail to gmail yahoo etc but cannot receive from server1.