[Pavel Nagaev]

Думаю, что NDR и DSN примерно одной природы, попробуйте уменьшить размер аттачмента для NDR

support.microsoft.com/kb/555231

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:
1)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).
2)Create a REG_DWORD value called MaxDSNSize.
3)Enter the value of the limits in bytes.
4)Restart the SMTP service.


Для NDR работает, возможно заработает и для DSN.