Why am I Receiving MAILER-DAEMON@yahoo.com Failure Notices?

Hi,

I realise that this is not an 'Apple-Related' question and I apologise for that.

Anyway, whenever I send an email from my Yahoo Account to my friend's Gmail Account, I keep getting a message from MAILER-DAEMON@yahoo.com saying that the email was unable to be sent.

Is there any specific reason that this happens?

How can I fix this?

Thanks,

Sandy-Wood