Cannot send emails to yahoo.com

Cannot send emails to yahoo.com

Hello, I got the error message below whenever I tried to send emails to someone registered at yahoo.com.  Everything was good until a few days ago this happened. Any idea how I can solve this? Thank you in advance.

Here is the error message that I got
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A message that you sent could not be delivered to one or more of its recipients. This is a permanent error.

ERROR CODE :554 - Message permanently deferred due to unresolvable RFC.5321 from domain; see https://postmaster.yahooinc.com/error-codes