I use Outlook Express 6 and Windows XP Pro Service pack 3 Build 2600.
Using outlook express I am unable to send a link. Address I was trying to send mail to was also a yahoo address. This was the explanation recieved:-

An unknown error has occurred. Subject 'Welcome | Real Tobacco', Account: 'Lyn Johnson', Server: 'smtp.mail.yahoo.co.uk', Protocol: SMTP, Server Response: '554 Transaction failed : Cannot send message due to possible abuse; please visit Yahoo! Help Form | Yahoo! Postmaster Help for more information', Port: 587, Secure(SSL): No, Server Error: 554, Error Number: 0x800CCC6F

Contacted Yahoo support for help and this was their response.

It appears that your email is being rejected by the domain of the
recipient's email address. Your best bet is to email a detailed
explanation of the error to the postmaster of the person you were

Please could someone advise what the domain would be as I thought that it would be Yahoo.

I would also like to advise that I can send the link okay if I log into my yahoo account online. Problem only occurs when using Outlook Express.