You need to request a 'Read recipt' for the mail. However, there is no guarantee that the recipient's mail system will send one as that is not mandatory and the recipient can choose whether to allow the receipt to be sent or not.
The ability to request the read recipt depends on what you are using to send your mail. If using webmail, BTYahoo doesn't support the facility but BTMail does. When composing your mail, click on the options button below the subject line and tick the 2 send receipts boxes. If you are using a mail client, you will need to look through the options of your particular client.
