From time to time our colleagues (all with email ID under tinoq.com, and DNS of tinoq.com points to mx.zoho.com) experienced the status 421 (Host not reachable) when sending email to each other.
This is error when I sent an email to Luke:
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 Thu, 27 Jun 2019 14:56:34 -0700
----- The following addresses had fatal errors -----
[Status: Error, Address: <
luke@tinoq.com>, ResponseCode 421, Host not reachable.]
Message will be retried for 4 more day(s)
Reporting-MTA: dns; mx.zoho.com
Arrival-Date: Thu, 27 Jun 2019 14:10:43 -0700
Status: 421
Action: failed
Last-Attempt-Date: 27 Jun 2019 21:56:34 GMT
Diagnostic-Code:
----- Transcript of session follows -----
Received:from [10.8.0.18] (ec2-34-214-64-27.us-west-2.compute.amazonaws.com [34.214.64.27]) by mx.zohomail.com
with SMTPS id 1561669846121311.4959217273465; Thu, 27 Jun 2019 14:10:46 -0700 (PDT)
Date:Thu, 27 Jun 2019 14:10:43 -0700
Subject:Oxford API requirement
Content-Type:multipart/signed;
boundary="Apple-Mail=_F0D31D47-1DCC-4665-B881-05A503A4FE0A";
protocol="application/pkcs7-signature";
micalg=sha-256
After some experiments, we found that it only occurred in email correspondence between two tinoq.com email accounts. That is, the tinoq.com email suffering this issue has no problem sending/receiving email to/from other email hosts, such as gmail or hotmail.
Could you please take a look into this issue? And inform us the root cause and help us fix this issue permanently.