Thomas,
Thanks for your post and your concerns about this module. I know that spam filters are a most confusing issue for everyone (hopefully people who spam too).
The information in your post is not exactly accurate. To prove the point, I have attached a screen shot of an example email sent from our demo page on our website. This page sends email through our correctly configured primary mail server that we host.
As you can see, I have sent an email test to my own hotmail account, and I have sent the email to be "from" an invalid email address at user@domain.com
You will also notice that the message was not stopped or blocked by the hotmail spam servers.
The reason for this is that although the domain.com email address is not valid, our mail server is... and it has a correctly configured reverse lookup against the primary IP address of the mail server. This resolves to valid lookups etc.
Additionally on our domain, and FYI on domain.com there are no SPF records advising the hotmail servers to reject mail unless it is sent from one particular server.
The from address is not relevant for spam blocking, unless that from address is listed on a filter black list. It is always the IP address of the generating mail server and subsequent reverse lookup DNS tables that determine how trusted a mail server is.
Now while some people reading this might take exception to some of this information (because I have very much simplified the issue). I have also not discussed content rating, return path tracking, and bayesian analysis etc. There is no need to mention it because hotmail clearly allow content to be delivered from this module when sent from a correctly configured mail server.
But don't believe me. Run a test form our module demo site and see the result for yourself.
P.S. We originally had the module set to send all mail from the email address of the site Administrator or Host settings. This caused a lot of problems, because there are a lot of DNN administrators who never bother to configure a valid email address in either their host or admin settings.
I hope you find this useful.