553 Relaying disallowed

553 Relaying disallowed

I have hosted my website www.grgpackersandmovers.com at parallel plesk on windows platform and mails are hosted at zoho.com.

I am getting mail sending failure message while sending email througn my .NET application. Please suggest me a solution.

Below is the notification message from mxtoolbox

220 mx.zohomail.com SMTP Server ready May 13, 2015 7:34:24 AM PDT

Test Result
SMTP Banner Check OK - 74.201.154.201 resolves to mx2.zohomail.com
SMTP Reverse DNS Mismatch OK - Reverse DNS matches SMTP Banner
SMTP TLS OK - Supports TLS.
SMTP Connection Time 0.718 seconds - Good on Connection time
SMTP Open Relay OK - Not an open relay.
SMTP Transaction Time 2.746 seconds - Good on Transaction Time
Session Transcript:
Connecting to 74.201.154.201

220 mx.zohomail.com SMTP Server ready May 13, 2015 7:34:24 AM PDT [655 ms]
EHLO MXTB-PWS3.mxtoolbox.com
250-mx.zohomail.com Hello MXTB-PWS3.mxtoolbox.com (mxtb-pws3.mxtoolbox.com (64.20.227.133))
250-STARTTLS
250 SIZE 25000000 [671 ms]
MAIL FROM:< supertool@mxtoolbox.com>
250 Sender < supertool@mxtoolbox.com> OK [655 ms]
RCPT TO:< test@example.com>
553 Relaying disallowed [671 ms]

MXTB-PWS3v2 4836ms