@Vinod Survase, Thanks for posting in Q&A.
Based on your description, I understand that you want to block the public email service via Intune or Defender.
Based on my researching , I found that there is no relevant setting in Intune or Defender. However, If you would like Intune to have such relevant settings, you can submit your feedback via the link below:
Thanks for your kind understanding and having a nice day!
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