Cannot receive some emails?

We are having trouble with some clients unable to send emails to our domain (Hosted with Outlook.com)

Some users are getting a bounced back email message along the lines of email cannot be delivered or recipient unknown at this domain.

Here is an example bounced back message:

"Your message did not reach some or all of the intended recipients.

Subject: RE: test email message

Sent: 5/09/2013 1:36 p.m.

The following recipient(s) cannot be reached:

'Mike Stevenson' on 5/09/2013 1:36 p.m.

There was a SMTP communication problem with the recipient's email server. Please contact your system administrator.

<DMSERVER.dm.local #5.5.0 smtp;550 Invalid recipient: <*** Email address is removed for privacy ***>>"

The issue is only with "some" clients, others have no problem reaching us at all.

Have checked out domain for blacklisting at mxtoolbox.com with no problems.

Is anyone able to shed some light on this?

Our domain is as above "epicsystems.co.nz"

Regards,

Jeremy Curran on behalf of Gerry at Epic Systems

Technical Consultant

Cambridge Computer Services (1972 Ltd)

Question about Outlook Express