email 550 50.1.1 telus

I host domains on servers outside the telus network.

Occasionally I get reports of certain domains who cannot send to anyone at one of my domains. Usually the senders use Telus as their ISP.  So, for example, no one at sendingdomain.com, hosted at Telus, can send to receivingdomain.com, hosted on my servers outside of Telus.

The returned message is always 550 50.1.1 and a sample follows.

Of course we checked all the obvious stuff like the email address is spelled right and the desitination mailbox is active and not full and we whitelisted the sending domain, which wasn't blacklisted anywhere as far as we could see.

It is only certain domains that can't send to our servers.  Is the problem likely at our end or theirs?

Solutions?

Thanks


Sample Error:

Date: Tue, 21 Sep 2010 13:46:30 -0400
From: Mail Delivery Subsystem <MAILER-DAEMON@mail124c0.megamailservers.com>
Reply-To: Mail Delivery Subsystem <MAILER-DAEMON@mail124c0.megamailservers.com>
Subject: Returned mail: see transcript for details
To: acl.pantera4@rigsat.com

The original message was received at Tue, 21 Sep 2010 13:46:29 -0400
from webmail@localhost

----- The following addresses had permanent fatal errors -----
lesley@hartoil.com
(reason: 550 5.1.1 <lesley@hartoil.com>... User unknown)
(expanded from: lesley@hartoil.com)

----- Transcript of session follows -----
.. while talking to localhost:
>>> RCPT To:<lesley@hartoil.com>
<<< 550 5.1.1 <lesley@hartoil.com>... User unknown
550 5.1.1 lesley@hartoil.com... User unknown

----- End forwarded message -----