Exchange 2010 receive messages threshold

Does some know what is exactly the threshold on Exchange 2010 to stop receiving emails when disk space is near to full.

I explain, last week i attended a customer issue related they were not able to receive emails (but they were able to send). Disk space was almost full, but it had 600Mb free. I´m supposing Exchange manages some threshold to stop receiving and avoid quick full disk problem, and allows users to send emails. It looks good, because services stay running and users don´t get errors.

After defragmentation of a small database i got 1.2 Gb free and users were able to receive emails.

Now, i need to inform my client what is the threshold used by exchange to stop receiving, i need to document it on my service report.

Thanks.

My need now is to fill a report of my service, and i