The problem is that user@ai is not globally unique. MIT users will have problems talk to user@ai when "ai" means Anguilla. The is a current security issue. If / when MIT stop using ai.mit.edu, "user@ai" will not longer mean user@xxxxxxxxxxx This will mean that any configuration file that has "user@ai" will now, suddenly, get a different meaning. This is a latent security issue.
If by "latent" you mean "so obscure that in the ten years that there's been A and MX records at TLDs nobody's been affected" I guess I agree.
R's, John _______________________________________________ Ietf@xxxxxxxx https://www.ietf.org/mailman/listinfo/ietf