Code (ASP/Javascript):

var myMail=Server.CreateObject("CDO.Message");
myMail.Subject="Subject";
myMail.From="info@domain1.blabla";
myMail.To="johndoe@test.blabla";
myMail.TextBody="This is a message.";
myMail.Configuration.Fields.Item("http://schemas.microsoft.com/cdo/configuration/sendusing")=2;
myMail.Configuration.Fields.Item("http://schemas.microsoft.com/cdo/configuration/smtpserver")="localhost";
myMail.Configuration.Fields.Item("http://schemas.microsoft.com/cdo/configuration/smtpserverport")=25;
myMail.Configuration.Fields.Update();
myMail.Send();

I've used this code on many other sites on the same server to complete
satisfaction.

Problem: on my new site it seems only 1 domain can be used for from
addresses. So anything@domain1.blabla causes the email to arrive, but no
other address or domain seems to work. I've received messages on several
domains that weren't the same as the working sending domain. The weird
thing is that the working domain doesn't have anything to do with the
website or domain this code runs on. It's on a different server.

Oh, and also: I don't have any spam filtering on the domains I receive the
test messages on, so it can't be that the messages are blocked depending on
the sender.