Undelivered Mail Returned to Sender

I am receiving an "Undelivered Mail Returned to Sender" when emailing an estimate to a client. I am able to copy and paste the email body & estimate PDF into my own email client and send that way. It appears to only be an issue when emailing from inside Zoho Invoice.

Here is a copy of the returned message. I've obscured the client's email and domain to avoid spammers:

This is a MIME-encapsulated message

--q2R3tFjv013175.1332820515/resolver2.sv4.zoho.com

The original message was received at Mon, 26 Mar 2012 20:55:13 -0700

from [172.31.251.172]

  ----- The following addresses had permanent fatal errors -----

<XXXX@XXXXX.com>

   (reason: 550-sv4-smtp2.zoho.com (resolver2.sv4.zoho.com) [72.5.230.67]:37560 is)

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

... while talking to XXXXX.com.:

DATA

<<< 550-sv4-smtp2.zoho.com (resolver2.sv4.zoho.com) [72.5.230.67]:37560 is

<<< 550-currently not permitted to relay through this server. Perhaps you have not

<<< 550-logged into the pop/imap server in the last 30 minutes or do not have SMTP

<<< 550 Authentication turned on in your email client.

550 5.1.1 <xxxx@xxxxx.com>... User unknown

<<< 503-All RCPT commands were rejected with this error:

<<< 503-sv4-smtp2.zoho.com (resolver2.sv4.zoho.com) [72.5.230.67]:37560 is

<<< 503-currently not permitted to relay through this server. Perhaps you have not

<<< 503-logged into the pop/imap server in the last 30 minutes or do not have SMTP

<<< 503-Authentication turned on in your email client.

<<< 503 Valid RCPT command must precede DATA

--q2R3tFjv013175.1332820515/resolver2.sv4.zoho.com

Content-Type: message/delivery-status

Reporting-MTA: dns; resolver2.sv4.zoho.com

Received-From-MTA: DNS; [172.31.251.172]

Arrival-Date: Mon, 26 Mar 2012 20:55:13 -0700

Final-Recipient: RFC822; xxxx@xxxxx.com

Action: failed

Status: 5.1.1

Remote-MTA: DNS; xxxxxx.com

Diagnostic-Code: SMTP; 550-sv4-smtp2.zoho.com (resolver2.sv4.zoho.com) [72.5.230.67]:37560 is

Last-Attempt-Date: Mon, 26 Mar 2012 20:55:15 -0700

--q2R3tFjv013175.1332820515/resolver2.sv4.zoho.com

Content-Type: message/rfc822

Return-Path: <zohoinvoicemailbounces+zcf=amanda=functionmarketing.com@zoho.com>

Received: from [172.31.251.172] ([172.31.251.172])

by resolver2.sv4.zoho.com (8.13.8/8.13.1) with ESMTP id q2R3tDjv013172;

Mon, 26 Mar 2012 20:55:13 -0700

Message-ID: <698429659.1332820513176.JavaMail.sas@invoice.zoho.com>

Date: Mon, 26 Mar 2012 20:55:13 -0700 (PDT)

From: Amanda Berlin <amanda@xxxxxx.com>

To: christy@xxxxx.com

Subject: Estimate from Function Marketing (Estimate #: EST-256)

Cc: amanda@xxxxx.com

Mime-Version: 1.0

Content-Type: multipart/mixed;

boundary="----=_Part_5829_347684101.1332820513166"

X-Mailer: Zoho Invoice Mailer

------=_Part_5829_347684101.1332820513166

Content-Type: text/html; charset="UTF-8"

Content-Transfer-Encoding: quoted-printable

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