Exchange error smtp;550 5.7.1 "Unable to relay for [e-mail]" when outsider trying to email user within my domain

[t***@bgop.org.uk]

Users from another domain cannot send emails with attachments to a user
on my domain, although they can send emails without attachments fine.

The full error (received by the users on the other domain) is:

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

Subject: Shows for weekend of 9/10/11 June 2006

Sent: 07/06/2006 10:01

The following recipient(s) could not be reached:

***@mydomain.com on 07/06/2006 10:05

You do not have permission to send to this recipient. For
assistance, contact your system administrator.

<othernetwork.co.uk #5.7.1 smtp;550 5.7.1 Unable to relay
for ***@mydomain.com>

Any ideas? I have seen plenty of stuff that would be applicable if my
users were sending the emails, but not when my users are the
recipients. Thanks.

[Daniel]

is the user currently employed/active? (dumb question i know but i had that
happen to me and it turned out the user had been terminated and was disabled
in the AD)

Post by t***@bgop.org.uk
Users from another domain cannot send emails with attachments to a user
on my domain, although they can send emails without attachments fine.
Your message did not reach some or all of the intended recipients.
Subject: Shows for weekend of 9/10/11 June 2006
Sent: 07/06/2006 10:01
You do not have permission to send to this recipient. For
assistance, contact your system administrator.
<othernetwork.co.uk #5.7.1 smtp;550 5.7.1 Unable to relay
Any ideas? I have seen plenty of stuff that would be applicable if my
users were sending the emails, but not when my users are the
recipients. Thanks.

[KingCronos]

Oh yes. Most certainly. She can receive emails from all others in our
organisation and out.

Post by Daniel
is the user currently employed/active? (dumb question i know but i had that
happen to me and it turned out the user had been terminated and was disabled
in the AD)

Post by t***@bgop.org.uk
Users from another domain cannot send emails with attachments to a user
on my domain, although they can send emails without attachments fine.
Your message did not reach some or all of the intended recipients.
Subject: Shows for weekend of 9/10/11 June 2006
Sent: 07/06/2006 10:01
You do not have permission to send to this recipient. For
assistance, contact your system administrator.
<othernetwork.co.uk #5.7.1 smtp;550 5.7.1 Unable to relay
Any ideas? I have seen plenty of stuff that would be applicable if my
users were sending the emails, but not when my users are the
recipients. Thanks.