User email validation problems - .nhs.uk not recognised as valid

I'm setting up a new site, and trying to add the initial users. All of the users have .nhs.uk email addresses.

E.g. firstname.lastname@sussexpartnership.nhs.uk

Drupal seems to have a problem with this, and says:

The e-mail address firstanme.lastname@sussexpartnership.nhs.uk is not valid.

How do I get Drupal to accept these as valid addresses?

Thanks!