Emails bouncing

Hi,

We use office 365 E3, we are having a lot of issues with mails bouncing when sending out.  Typically I get the following bounceback:

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

      Subject:    FW: ******* - Online Reviewing has started - ********

      Sent: 16/12/2013 10:26

The following recipient(s) cannot be reached:

      '***********@nnuh.nhs.uk' on 16/12/2013 10:26

            This message could not be sent. Try sending the message again later, or contact your network administrator.  Error is [0x80070005-00000000-00000000].

I am having this issue sending from shared mailboxes but our normal inboxes work fine.  Please can you advise on how to fix this?

Cheers

Greig