NDR error messages

[AnnieBlueEyes]

We communicate regularly with one of our managers who uses a dsl connection and who has an e-mail address with pacbell.net. We have an Exchange 2000 server and our own e-mail domain.

Today the manager received a failure notice when she sent an e-mail to one of our users on our domain from MAILER-DAEMON@yahoo.com the message read:

Connected to 199.171.54.248 but sender was rejected.
Remote host said: 553 <xxxx@pacbell.net>...User unknown

When one of the users on our domain sent an e-mail to this manager today, she received an undeliverable notice from the System Administrator stating:

The following recipient(s) could not be reached:

xxxx@pacbell.net on 10/17/2005 8:43 am
553 <xxxx@pacbell.net>...user unknown

The user on our domain also received this message for another pacbell.net e-mail address.

The same user on our domain also received a returned mail: service unavailable from MAILER-DAEMON@uu.net (which is our service provider) with the following:

The original message was received at Mon, 17 Oct 2005 16:35:44 GMT from
server.fga-net.com [65.200.44.145]

----- The following addresses had permanent fatal errors -----
<xxxx@juno.com>

----- Transcript of session follows ----- ... while talking to
mx.nyc.untd.com. [64.136.20.83]:
<<< 550 Access
denied...04d52ded0535790ccca928ad95f59518dc9889a148ddc8c8dd48d9618848f8684d.
..
... while talking to mx.lax.untd.com. [64.136.28.83]:
>>> QUIT
<<< 550 Access
denied...4a55052d252d41c429ed45edf5eda93535c1dd343d8d79798d3d54e1b53d39a150.
..
554 <xxxx@juno.com>... Service unavailable

I don't think this is a problem on our end, but if it is, I need to know how to remedy the problem, or maybe someone can explain how to determine where the problem is stemming from.

Any tips would be greatly appreciated! (I hope I explained this properly so you can understand it!)

Thanks, Anne