Yahoo have extremely strict criteria for accepting email and often defer e-mail if they think that it might be spam or if they are busy - they call this 'de-prioritising email'. Unfortunately they have a very poor system that often defers perfectly acceptable email. Yahoo also run the e-mail system for BT so you may get the message sending to BT email addresses too.
It is widely known in the industry that Yahoo have this poor system and we always recommend that you do not use Yahoo for email purposes because of the amount of email that they block or defer.
Please note that there is nothing that we or any other web hosting company can do about this. Our mail servers will keep trying to deliver your message and Yahoo should eventually accept it.
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