[Spiral]

As for what the web development company told your client ...

It could be that they are calling proprietary external object or function
libraries stored on their server they don't want to release and might have
some technical reason for asking to host the client. Then again, they could
have told the client they needed to host their site just because they want
to be the client's host which is a bit unethical but not unheard of in the
competitive hosting business. Either way, I'm always very skeptical
about any programmer that say "you must host with us" for any reason.

Your client should also consider what happens if the web developer suddenly
goes out of business. If they are keeping hard line control of the web site
(and thus the content developed) then the client may be in very serious
trouble if something happens to the web developer. Web developers that
are more "on the level" usually won't have any problems with accepting
your money for the development job and you taking the files with you to
wherever you want to go host them.

Now regarding the original question and what to do as far as email goes ...

You could reduce their account to use the x3mail template and set them up
for mail only and on the DNS side, just update their zone file to have the
IP address of the web developer's server for the base domain and "www" while
you setup your IP for the "mail" subdomain and set the MX record on the
domain to go to the "mail" subdomain (with your IP) for mail handling.


(If the web developer tells the client they must change the DNS servers on
their domain to the developer's own DNS servers, that should be a red flag
that all they are really trying to do is steal the client because there is really
no technical reason that it would be necessary to change the DNS servers
but doing so could effectively sweep control out from under you so that is
something you should be wise about and would probably be the developer's
next recommendation to the client)