Whenever a message from Zoho is sent to Yahoo, I am getting a warning on delivery delays and one of the lines indicate the following status:
----- The following addresses had fatal errors ----- [Status: Error, Address: <...@yahoo.co.in>, ResponseCode 421, , 4.7.0 [TS02] Messages from 72.5.230.103 temporarily deferred due to user complaints - 4.16.56.1; see http://postmaster.yahoo.com/421-ts02.html]
Message will be retried for 4 more day(s)
A quick research on Yahoo brought the following URL: