Why do I keep getting Delivery Status Notification Failure from "postmaster@hotmail.com"?

I am trying to email my mum and attach some photos, I know her email address is correct as I have used it loads of times and I am just going with the one in my address book so I know there are no typos, so its not that the address is wrong. Also I sent the same email to my two sisters, one with yahoo email went fine and the other sister also has hotmail and that failed too?? I have yahoo email and Im trying to send to my mums hotmail.co.uk address any ideas? Just to re-itterate they are still current email addresses and are definatley correct!