Hi @Margaret1M
Welcome to our Community Help Forum 🙂
Thank you for making your first post regarding your issue where emails sent to your sisters Tiscali address bounce back.
Unfortunately there is a known issue where Tiscali email addresses do not receive emails sent from Virgin Media, Talk Talk are currently looking into the problem. For more information, please take a look at the thread @Graham_A linked above 🙂
Have you tried sending the emails to your sister via the Virgin Media Mail webmail page instead? https://mail2.virginmedia.com/
Hope this works in the meantime!
Thanks,
Serena