I'm having issues with yahoo mail too, I send out newsletters to a myriad of email addresses, gmail gets delivered no problem, as does my own domains' mail - but yahoo cannot be delivered...
the error reads:
HTML Code:
MailEnable: Message delivery has been delayed.
Message is waiting at eurofighter.eukhost.com for delivery to mx2.mail.ukl.yahoo.com.
The message will be retried for another 29 hours.
Reason: Mail Server for yahoo.co.uk could not accept your email at this time. MailEnable will keep trying to deliver this message and will notify you of any progress.
Is this a yahoo problem? or a server problem this end?