Non delivery reports(NDRs) are sent to a recipient when a message he sent could not be delivered by the mail server. When Exchange server sends such a message it attaches the original message to the NDR message. This behaviour can be altered.
A system administrator may specify a size limit for original messages that will be attached to the NDR message. Messages that are larger the the limit specified will not be attached to the NDR message.
The following steps describe the procedure for configuring the limit:
Open the Registry Editor and point it to the following path: HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\SMTPSVC\Queuing (if the Queuing key does not exist create it).
Create a REG_DWORD value called MaxDSNSize.
Enter the value of the limits in bytes.
Restart the SMTP service.