External emails not coming through

Hi

We have some external customers tying to email us. I see in our domino server logs under Mail Routing and Misc Events the following entry when his email is briefly accepted by the domino mail server.

SMTP Server: Error storing received message

The sender then received a non delivery report back saying...

Delivery has failed to these recipients or distribution lists: user@user.user.com

An error occurred while trying to deliver this message to the recipient's e-mail address. Microsoft Exchange will not try to redeliver this message for you. Please try resending this message, or provide the following diagnostic text to your system administrator.


The following message to <Ken.smith@business.com> was undeliverable.
The reason for the problem:
5.3.0 - Other mail system problem 554-'Error writing message to safe storage; message could not be stored to disk'



Any ideas?