Email routing problems

[TomT]

Hi.

Bit of background...

I host my own mailserver, and have the correct 'A' and MX records 
created, pointing to my IP Address and emails work fine..

I have just arranged to have my domain name hosted at a new ISP, I'm 
going to continue doing the DNS.

With in my ISP's cPanel, they have set an MX Record pointing to 
domain.co.uk, yet I have an MX record of mail.domain.co.uk.

If I send from an outside address an email to user at domain.co.uk, the 
email comes into my server.

Problem:

At my new host I have setup a phpbb forum, in the configuration I have 
setup smtp -> mail.domain.co.uk , and set a valid email address.

If I use any of the email featues (request password) the email doesn't 
go to my server, its stays on my ISP's webmail server. I seems to be 
resolving internally.

My ISP has advised a ping or telnet to mail.domain.co.uk fails with host 
unknown.. Yet this works fine from any other system.

The ISP offers cPanel and is linux based.

Can anyone offer some advise or suggestions. ?

Thanks