Calendar Invitations & Responses not getting through

I am not able to receive an invite nor acceptance; see below return message sent by an invitee:

_____________________________________________
From: postmaster@w14d.comcast.net [mailto:postmaster@w14d.comcast.net]
Sent: Monday, April 13, 2015 5:41 PM
To: Wright, J (Tax&Accounting)
Subject: Undeliverable: Accepted: evaluate self-insured manual

Delivery has failed to these recipients or groups:

byron@dbs-ca.com
There's a problem with the recipient's mailbox. Please try resending the message. If the problem continues, please contact your helpdesk.

Diagnostic information for administrators:

Generating server: healthdatasystems.com

byron@dbs-ca.com
#< #5.2.0 smtp;554 5.2.0 STOREDRV.Deliver.Exception:DelegateUserValidationException; Failed to process message due to a permanent exception with message Delegate user validation failed for user: Sarah Martinez. DelegateUserValidationException: Delegate user validation failed for user: Sarah Martinez.> #SMTP#

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From: <j.wright@thomsonreuters.com>
To: <byron@dbs-ca.com>
Subject: Accepted: evaluate self-insured manual
Thread-Topic: evaluate self-insured manual
Thread-Index: AdB2MeOSDnYPDtgtR+i8Ktml05KvpwAAAAeQAAAFF0AAABwM0A==
Date: Mon, 13 Apr 2015 21:40:06 +0000
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