Not registered for iMessages

My wife has a problem with her iPad mini.When she tries to send an email the name sending it to turns red and the message "Not registered with iMail".
Can anyone help?.
Regards

Go to Messages in Settings and make sure that iMessage is enabled and the appropriate IDs are registered.

Hello David
Thanks for info. But how does she register contacts with iMessage.
Tony

To register herself to use iMessage go to Settings > iMessage > Send & Receive. You will need your Apple ID and can then add pine numbers and email address as necessary.

To find people to send messages to add them to the Contacts with a phone number of email address. iMessage will only send to people with an iDevice.

When I had an iPhone I had major problems with iMessage. As I had plenty of texts I just turned iMessage of and used traditional SMS instead.

The biggest issue I had with iMessage was when I had poor mobile signal. It would decide that as I had a flaky GPRS connection that it would use iMessage and then 10 minutes later it would fail. But you couldn't easily force it to SMS - and SMS would work fine because it doesn't need a data signal to use it.

Either that or just use one of the other cross platform messaging solutions (like whatsapp or Facebook messenger or whatever) and toss the iMessage rubbish in the bin.