Not receiving emails on Zoho with ResponseCode 421, Host not reachable

Hello everyone,

I'm not able to receive emails from anyone from last Friday. I tried to send emails and it seems working.

I sent an email to myself and the error I received was ResponseCode 421, Host not reachable.

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 Sat, 12 Aug 2017 15:51:33 -0700
from osama.abouelkhir@smartyotta.com [osama.abouelkhir@smartyotta.com]

----- The following addresses had fatal errors -----
[Status: Error, Address: <osama.abouelkhir@smartyotta.com>, ResponseCode 421, Host not reachable.]

Message will be retried for 4 more day(s)

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

Received:from mail.zoho.com by mx.zohomail.com
with SMTP id 1502575567781112.42324742646042; Sat, 12 Aug 2017 15:06:07 -0700 (PDT)
Message-ID:<15dd87ba3a3.db0c8070131574.1292069438475157850@smartyotta.com>
Date:Sun, 13 Aug 2017 02:06:07 +0400
From:Osama AbouElKhir <osama.abouelkhir@smartyotta.com>
User-Agent:Zoho Mail
To:<osama.abouelkhir@smartyotta.com>
Subject:(No Subject)
Content-Type:multipart/alternative;
boundary="----=_Part_315432_1781482228.1502575567780"