Email getting delayed and not reaching the intended recipients..

Email getting delayed and not reaching the intended recipients..

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 Tue, 22 Dec 2015 21:18:47 -0800
from  ashish@ufony.com  [ ashish@ufony.com ] 
Email shows these messages as sent but the receiver are either not getting it or getting it too late.. happening intermittently for many IDs. can you please check and fix as our work is now getting affected ?
   

Log below for one such email..

----- The following addresses had fatal errors -----    
  [Status: THROTTLED, Address: < xxx >, ResponseCode 421, 4.7.0 Temporarily Deferred]

Message will be retried for 4 more day(s)

   ----- Transcript of session follows -----    

Received:from [192.168.2.5] (103.51.153.14 [103.51.153.14]) by  mx.zohomail.com
with SMTPS id 1450845209067291.1233423270894; Tue, 22 Dec 2015 20:33:29 -0800 (PST)
Message-ID:< A88FF3AB-C004-4C9F-80A4-7D1B60759A22@ufony.com >
Date:Wed, 23 Dec 2015 10:03:17 +0530
From:Ashish Chaturvedi < ashish@ufony.com >
To:Vidya Nair < xxx >
Cc:Susshil Nair <x xx >
Subject:Re: SchoolDiary
Content-Type:multipart/alternative; boundary="Apple-Mail=_0A2E3DC3-67A4-494A-B4D6-3E7438B1503C"