Is the email in the client‘s “Junk” folder?
If you know the recipient, ask if your message got sent to their junk folder by accident. If it was, tell them to add your email address to their "trusted senders" list, or their address book. That usually helps prevent you from being tagged as a spammer in future campaigns.
Delivery March 8, 2019
Why did Yahoo send my emails to the bulk mail folder?
Our white-list agreement with Yahoo does not guarantee perfect delivery. Yahoo reviews messages for server reputation, message content and sender address. Be sure to use our spam check to avoid message flags. Your message may also be flagged if you have received spam complaints in the past from the same sender address.