yahoo declines to recognize my contact

----- Forwarded Message -----
From: "[undisclosed email]" <[undisclosed email]>
To: [undisclosed email]
Sent: Thursday, 23 March 2017, 15:41
Subject: Failure Notice
Sorry, we were unable to deliver your message to the following address.
<[undisclosed email]>:
No MX or A records for hemma.be
Which is curious because we have communicating for several years with no trouble!