Exchange error: #501 unacceptable mail address

I have a user in our organization that sends out an email to a user and gets the following bounceback.
Most other emails are working fine. Looking how to read the fine print for troubleshooting.
Thanks in advance.
Diagnostic information for administrators:

Generating server: MAIL.seacoastmedical.local #501 unacceptable mail address ##

Original message headers:

Received: from MAIL.seacoastmedical.local ([fe80::4d2f:98fa:4c4f:616b]) by
 MAIL.seacoastmedical.local ([fe80::4d2f:98fa:4c4f:616b%13]) with mapi id
 14.02.0318.001; Fri, 24 Aug 2012 10:00:22 -0500
From: user <>
To: 'Tony User' <>
Subject: RE: test
Thread-Topic: test
Thread-Index: AQHNggkp3T0LuCVvjUC7DCbwR2OmSJdpDkdQ
Date: Fri, 24 Aug 2012 15:00:22 +0000
Message-ID: <BF3933C7D676B54D913F3D99091655181DA4B1@MAIL.seacoastmedical.local>
Accept-Language: en-US
Content-Language: en-US
x-originating-ip: []
x-tm-as-product-ver: SMEX-
x-tm-as-result: No--29.796500-8.000000-31
x-tm-as-user-approved-sender: No
x-tm-as-user-blocked-sender: No
Content-Type: multipart/alternative;
MIME-Version: 1.0
Who is Participating?
Simon Butler (Sembee)ConsultantCommented:
Your server is listed as the GENERATING Server because the remote server has rejected the message at the point of delivery. It hasn't allowed the message to be delivered.
A lot of mail admins will see that NDR and use it as an excuse to point to your server as the source of the problem, which is not the case.

I also know it isn't your server because the NDR isn't an Exchange NDR. The domain you mention - - isn't an Exchange server, so the NDR has to be from them.

Manpreet SIngh KhatraSolutions Architect, Project LeadCommented:
Check this article ... are you using some Firewall ?

501 unacceptable mail address

Possible reasons and solution
The most common reason for this error is failure to assign a valid email address to the mandatory From property:
Mail.From = ""

This error may also occur if a name instead of email address is specified for this property.

Another reason is invalid syntax in an email address passed to any of the methods AddAddress, AddCC, AddBcc and AddReplyTo.

- Rancy
Simon Butler (Sembee)ConsultantCommented:
That is a problem with the remote server, nothing you can do about it from your end. They are rejecting it for some reason, and only their administrators can tell you why.

Probably an antispam measure of some description.
They are rejecting it at the point of delivery, which is why your server is listed as the generating server.

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Todd GerbertIT ConsultantCommented:
This KB article  suggests error 501 indicates an attribute is too long (though that may not be the case here).

So, I assume mail.seacostmedial.local is your server? What version of Windows and Exchange are you using?

Can this user send mail to this recipient using other means, e.g. Outlook Web Access, Outlook Mobile, SMTP, via your server?

Can this user send mail to the recipient if he's logged onto a different workstation?

Can other users in your organization successfully send mail to the same recipient?

Is this particular recipient represented by a Mail Contact in Active Directory?
Todd GerbertIT ConsultantCommented:
Oh, never mind. ;)

Simon: how can you tell it's the fault of the remote server, since mail.myorg.local is specifically identified as the generating server?  Or just based on experience?
dblatchfordnetworkAuthor Commented:
mail.seacoastmedical.local is my mail server.
No other users can email this user or other users from our mail server to the domain.
We get the same result if we use owa.
I am also curious, it looks like our mail server is doing the rejecting here, and the bounce back is immediate.
Thanks in advance.
dblatchfordnetworkAuthor Commented:
Thank you for the response and answer.
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