I seem to be getting Quarantine Notifications to a large number of unrecognised recipients at our domain.
Exchange accepts the emails because they are addressed to our domain but cannot then find a recipient.
I used to get notifications to a single address then notifications stopped for a while and now this.
Any ideas?

Previously quarantine emails for the catch all were rolled up in to a single email. However the links to log in to IronPort for some reason have to have a URL that contains a list of all the recipient email addresses for the emails being quarantined. This got so huge that it broke - see Huge Spam Quarantine Notification Messages

I suggest you reconfigure the way you have spam handled.

Create a new mailbox(s) (I have one for each domain hosted) called domainname_spam. Then in MMM spam settings choose Move to an existing Mailbox instead of quarantine and select the dedicated spam mailbox. You can then check that for false positives using IMAP and either forward them or, if you also have access to the mailbox the genuine email should have gone to via IMAP using something like Thunderbird you can drag and drop the false positive to the correct mailbox.

Thanks John. I'll have a look at that when I get time.

Edit:..and thanks Jim for showing me the error of my ways

Not an error - I just thought you were more likely to get help here   

I'd started my reply before you moved the topic so it didn't make any difference!

I placed it in business in anticipation of a more knowledgeable reply 

I should have known the knowledgeable are everywhere on this forum

Thanks once again.