Messages just appear to be falling into the realms of Cyber space with this error, when running the tracking console in ESM you simply see the message:
SMTP: message submitted to categorizer
I've found this to be two seperate issues, most commonly issue one will work.
It is Anti Spam that is causing the issue - submitted to categoriser is not a standard entry in exchange, meaning its been sent out of exchange to be sorted for spam. I can't stress enough that checking your quarantine or your instant delete settings will solve this issue.
It's an issue with the listing in exchange for the GC, follow the below steps to resolve.
Open ESM, expand "Servers", right click on the server and select "Properties", click "Directory Access", select "Global Catalog Servers" from the dropdown and uncheck "Automatically discover servers" (Bypass the warning) and add the GC (Servername in SBS environments) manually.
Then Restart exchange services to find that all mail begins to deliver to store as per normal.