TLS currently unavailable

TLS currently unavailable

I keep getting "TLS currently unavailable" when sending mail to a particular email address.  When I send a message to this address using iCloud mail it doesn't complain about TLS.  Is there an incompatibility of the TLS/SSL versions between zoho mail and certain other email systems?

See below for the warning message.  It times-out after 4 days and returns a message saying the email is undeliverable.
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 Wed, 01 Jan 2020 10:29:21 +0100
from ************
   ----- The following addresses had fatal errors -----    
  [Status: Error, Address: <**************>, ResponseCode 454, TLS currently unavailable]
Message will be retried for 4 more day(s)
Reporting-MTA: dns; mx.zohomail.eu
Arrival-Date: Wed, 1 Jan 2020 08:44:09 +0000
Original-Recipient: rfc822; *********
Final-Recipient: rfc822; ***********
Status: 454
Action: failed
Last-Attempt-Date: 1 Jan 2020 09:29:21 GMT
Diagnostic-Code: