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!

Answer 1

I believe that those records are obsolete.
They were trying to replace some SMTP commands like VRFY and EXPN.

Answer 2

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.