sendmail abuse by names.co.uk

Hi there,

I'm working on a site for a friend of mine and he has an email response system built in to the php using the mail function.

All emails are being sent with an envelope-sender that is an email address that is not real - Names.co.uk won't fix it and say it is to make sure that emails appear to come from the same server as the email address sending.

Has anyone else ever come across something similar and is there a way round it?

Thanks,

Rob.