Suddenly no emails able to be sent or received

Suddenly no emails able to be sent or received

All was working well for myself and my users however, since first thing this morning we are not receiving emails and are not able to send them.

When sending, we get the following error message '550 Administrative prohibition'. We have made no changes to our systems.

Also, people are unable to send us mail, they get an 'unable to send' message.

Please can you assist as this is urgent. We are unable to do business until this is fixed. Can't get through to Support - frustrating to say the least.

Cheers
Christine