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The email system is dependent on co-operation. WHat a pity Microsoft haven't worked out what that means. A rational response would be for MSPs to organise a back channel for sorting out problems. If MS have a problem with another MSP then use that channel to raise it and let that MSP sort it out. Blocking the entire SP should only be the last resort. Of course it may well be that such an arrangement is in place and has failed to resolve the matter so an additional action should be to announce that this mechanism in use. In that way innocent users of both services (yes, Microsoft, it's your customers who are rendered unable to receive mail) at least know what's happening and can take avoiding action.