Permission denied message when trying to update a Webui created ticket via email

Hi all

I had a complaint from a user trying to update a ticket via email which was created Using the webui they received a message back saying

Body of email

Permission denied

In the logs I have

Sep 11 14:12:03 XXXX RT: Permission Denied (/opt/rt3/bin/…/lib/RT/Interface/
Sep 11 14:12:04 XXXX RT: Could not record email: Message not recorded: Permission Denied (/opt/rt3/share/html/REST/1.0/NoAuth/mail-gateway:75)

Does anyone have any idea what is wrong, I have other client who log tickets by email all the time as they work ok.

Thanks in advance Simon