Strange hosting problem.

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@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.

The ISP offers cPanel and is linux based.

Can anyone offer some advise or suggestions. ?

Thanks