I believe that those records are obsolete.
They were trying to replace some SMTP commands like VRFY and EXPN.
DNS Resource Record Type Field (MB, MG, MR)
Question
What is usage of MB, MG, MR types in a Resource Record?
Some information in Here, but I can't understand their usage in real world!
Solution
OTHER TIPS
According to the IANA DNS parameters list they were EXPERIMENTAL
RR types proposed (before the DNS properly existed) in RFC 883 and specified in RFC 1035.
AFAIK, they've never seen any significant real-world deployment.
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