I don’t have any previous experience with Exhange, our domain name was not yet registered, a week ago I have installed Exchane 5.5, and I succeded in configuring the organization, the site, Internet Mail Service and the mailboxes, and I succeded in sending mails. But after a few days I noticed a terrible uploading activity in our Internet connection. From the network monitor installed on the Exchange server machine, I found that all the packects are SMTP, in brief it was a junk mailer who used our Exchange to send his junk mails. All I could do is to stop the Internet Mail Service and everything went back to normal.
My question is how he was able to do that and in the first place how he could know about us (specially that our domain was not yet registered, so we can only send mails), second how can I prevent this from happening again.
I hope you can help me with this, and if this is the right way to send my inquiries please inform me with the right one.
Thanks in advance.
Best regards