Emails Resending 10, 20, 30 times

Emails Resending 10, 20, 30 times

I sent an email yesterday using Apple Mail (my usual email client) to someone and then a few minutes after sending it I received a warning email from Zoho saying the following:

This message was created automatically by mail delivery system.
THIS IS A WARNING MESSAGE ONLY.
YOU DO NOT NEED TO RESEND YOUR MESSAGE.
The original message was received at Mon, 14 Sep 2020 18:12:43 -0700
from (my email) (my email)
   ----- The following addresses had fatal errors -----    
(receivers email)

Message will be retried for 4 more day(s)


Reporting-MTA: dns; mx.zohomail.com
Arrival-Date: Tue, 15 Sep 2020 12:19:05 +1200


Original-Recipient: rfc822; (receivers email)
Final-Recipient: rfc822; (receivers email)
Status: 421
Action: failed
Last-Attempt-Date: 15 Sep 2020 01:12:43 GMT
Diagnostic-Code: Read timed out

I then received a panicked email from the person saying that the email I sent has come through on their end between 10 and 20 times!

This is the third time I have had this issue over the last two years always with the same process.

How can I stop this from happening? I am loosing business because of this.