Ultimately, the issue resides with the recipient’s email server and can’t be fixed by yourself.
Before contacting the intended recipient via a different method to let them know of the issue (eg, by using a different email address or calling them by phone), you may first wish to check the message at the very top of the bounce-back email to confirm that you are sending the email to the correct address;
“Your message to [recipient email address] couldn’t be delivered.”
If this address is incorrect or not the address you were expecting, you may simply need to re-send the email to the correct email address.