I registered a .tk domain, called him beep.tk (not my actual domain)
I registered beep.tk with custom DNS, so I got an account on freenom.com
From there, I started setting up MX records using the freenom toolbar, pointing them to my IP address of the mail server, and it worked: postmaster@beep.tk will send / receive email just fine
About the necessary traffic rules: I have beep.tk for several months, and I had no contact about the recall, but it may help that I used it as a website.
Although only I and few others knew beep.tk, I had more than 1199 hits in one month, I think from scanners such as google, since I registered my mail server with them
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