Event ID 4006 / 4007 - The SMTP verb which caused the error is

Hi,

I'm experiencing problems with my w2k3 standard etd.
IIS has been installed and several websites are running.

There has been a virtual smtp server installed with relay access
granted to a few ip adresses AND the localhost.
Access has been given to everybody (anonomous).
So, no mailserver has been installed on this server and i'm getting
several messages in the eventlog.
a few examples:

Message delivery to the host 'xxxxxxxxxx' failed while delivering to
the remote domain 'xxx.com' for the following reason: An SMTP protocol
error occurred.
The SMTP verb which caused the error is 'MAIL'. The response from
the remote server is '451 4.1.8 Domain of sender address
noreply@domain.com does not resolve.
Event id = 4007

or

Message delivery to the host 'xxxxxxxxxx' failed while delivering to
the remote domain 'xxxxx.com' for the following reason: The connection
was dropped by the remote host.
Event id = 4006

Does anybody knows 'what' the problem is. Spam? Or perhaps a problem
in the software?