[exim] Exim 3.36 cannot handle deliver a lot emails

Hi

I am using Exim 3.36 under Debain Linux. I also using Exiscan,
SpamAssassin 3.0.3, clamav with Exim.

Now I wirte a MS.Net batch email client progream which try to send
about 2000 emails through my Exim server.

The first 30s email works fine. But after that, I get a error:

Exception: System.Runtime.InteropServices.COMException
Message: The transport failed to connect to the server.

It seems like my Exim server reject deliver email for me.

I am sure Exim mail server can handle 2000 email in once. Is any
config I need to do with my exim server?

Cheers,

Tao Lin