Exchange Server or NS filtering email addresses?

My Notification Services (NS 2.0 SP1) application works fine, except one big problem:

I am using a built-in SMTP provider, and notifications are delivered ONLY to those subscribers who have email addresses within our company's domain.

That is, all subscribers with addresses like someone@.ourcompany.com always get their notifications, whereas any other email addresses do not work. I tried my gmail.com, aim.com, and optonline.net email addresses, and none of them ever received a notification, not even once.

As soon as I change a particular subscriber's email address from whatever to anybody@.ourcompany.com (whether I do it via SMI or directly in the NS subscribers table, does not matter), it works again.

So, something is filtering emails based on the recipients' addresses.
I do not know for sure whether it's done by NS or by the SMTP server, but the situation is the same regardless of whether I use my local machine's SMTP server or our company's MS Exchange Server 5.5 (I specify it only in the InstanceConfiguration.xml file).

Could someone point me in the right direction as to where to look for what's causing this kind of filtering, please?

Thank you very much!Answering my own question...
Here's what the Exchange Server 5.5 returns in an exception message:
The server rejected one or more recipient addresses. The server response was: 550 Relaying is prohibited.
It's a spam-filtering -related feature.