“Greylisting is a method of defending e-mail users against spam. A mail transfer agent (MTA) using greylisting will "temporarily reject" any email from a sender it does not recognize. If the mail is legitimate, the originating server will try again after a delay, and if sufficient time has elapsed, the email will be accepted.” (Wikipedia contributors. (2018, May 15). Greylisting. In Wikipedia, The Free Encyclopedia. Retrieved 14:26, November 15, 2018, from https://en.wikipedia.org/w/index.php?title=Greylisting&oldid=841400168)
It would be great and a big reason less to use mail gateway in front of Domino for receiving mail from internet.