Finally I managed to stop the SMTP hacking, generating tons of "SMTP Server: Authentication failed for user ..."-messages
Have you ever seen a bunch of these in your Domino log-database?
If so, you are the victim of attempted SMTP Hacking. It means that someone is trying to log on to your server via the SMTP protocol. SMTP was primarily constructed for functionality and not so much security
Unnecessarily to say, it is a waste of cycles to have all this traffic hit your server, not to mention what could happen if they ever succeed entering the system! Obviously the perpetrators uses some sort of directory attack trying out both known and valid mail addresses, in addition to all sorts of other names.
Read on to see how I finally was able to stop specified IPs from even reaching the Domino server (hint, the Windows Firewall comes to rescue!)
Update March 7th, 2017: David Brown over at his site https://port1352.wordpress.com (love the port1352-name!) took the time to chime in and tipsed me about his blog-post Disable SMTP-AUTH To Stop Relay Hackers In Their Tracks . I had actually seen this post too, but I could not get my head around that it would could actually be better to turn off SMTP-authentication in order to have better security. I asked a question on his post, and he posted a thorough answer to why this might be a good idea here. Thanks David for taking your time to answer this question!