If you or your company sends out bulk email to your customers frequently then you may have come across a situation where Yahoo may have blocked your emails from reaching its users. Most of the time the problem occurs when customer reports your email to Yahoo for spam. Once a specific number of people report your email to Yahoo for spam then Yahoo blocks your emails from reaching its users.
The bad news is that Yahoo does not notify you that your emails have been blocked and the only way you can check this is by checking your mail logs by logging into your WHM panel or you can use the Trace Email option in your cPanel. Just enter the email address of the customer and the system will display the message given out by Yahoo.
The message will most likely be in the format given below
SMTP error from remote mail server after MAIL FROM: SIZE=501178: host mta6.am0.yahoodns.net [18.104.22.168]: 421 4.7.0 [TS01] Messages from [xxx.xxx.xxx.xxx] temporarily deferred due to user complaints
Yahoo itself states on its website that this problem is temporary and you should try sending email to Yahoo users after a gap of 4 or more hours however if the problem has not resolved then they ask you to submit a Yahoo Bulk Mail Sender form on their website.
Once you complete the form you will receive an acknowledgement from Yahoo.
I received a reply from Yahoo after 36 hours which stated that they have removed the restriction and now our email would be delivered to Yahoo users.
Hope the above helped.