Hi @Sanket Khot ,
Hope you are doing well.
As per my knowledge, we cannot get event details of external/guest user, or any user that does not actually have an Exchange Online mailbox (within your own tenant). The only way around this would be to have AAD app and the external user (or an admin from his organization) consent to your app, thus add it to their own tenant and grant the relevant permissions. A mailbox will still be required.
Hope this helps.
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