Mail not getting to Yahoo email addresses

[nic.lk]

I operate an online store. When we update customers orders an email goes to the customer confirming the different stages. Yahoo mail account holders do not seem to be receiving these mails.

What could be causing this?

Thanks
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[chandani099]

I have a mail server. It has a big queue. Reason for the queue is yahoo deferred mails. Most of mails are deferred for no reason. Even I have notice that most of my registration Yahoo mails are not coming . So I quit using Yahoo.