Are you a TalkTalk Consumer broadband or MailPlus customer? If so, is the tiscali mailbox in your MyAccount when you view your email addresses?
To link this topic to your TalkTalk service please check your Community Profile includes in Personal Information (Click here):
Your name, current TalkTalk landline 'phone and alternate number (mobile preferred). Add full address with postcode (location). Then scroll down to Private notes to add your tiscali email address, notes and references etc and then save changes.
Compose and send the same mailbox a message to see if that sends without error, is copied to the Sent objects folder and then arrives and stays in the Inbox.
If the send fails then let us know what the full error message is.