Can anyone tell me what this message means?

>[log in to unmask] sent a DISTRIBUTE job that was rejected for
>the following reason:  "You are not allowed to  use inter-server DISTRIBUTE
>control keywords". Job 63308 ("X-SUPD JOB") transferred to [log in to unmask]

We suddenly began getting these regularly a few weeks ago.  We had only one
subscriber at this domain, who didn't respond when emailed, so we removed
the subscription, but the messages continue.  We tried sending email to
both [log in to unmask] and [log in to unmask], with the
following response:

>This Message was undeliverable due to the following reason:
>
>Each of the following recipients was rejected by a remote mail server.
>The reasons given by the server are included to help you determine why
>each recipient was rejected.
>
>    Recipient: <[log in to unmask]>
>    Reason:    <[log in to unmask]>... Invalid recipient - not
>registered
>
>    Recipient: <[log in to unmask]>
>    Reason:    <[log in to unmask]>... Invalid recipient - not registered

What else should we try?

Sherry Beauchamp