Message=Code: ErrorInvalidUser Message: The requested user '5f1fb9dd-2fc2-4f4d-8fa4-ebd144432b9e@eb0eeae1-e81a-4a23-a4e4-ca8688e0a18a' is invalid

Sending mail using graph API from Azure function like below

await graphClient.Me.SendMail(message, true).Request().PostAsync().ConfigureAwait(false);

Getting Error: Message=Code: ErrorInvalidUserMessage: The requested user '5f1fb9dd-2fc2-4f4d-8fa4-ebd144432b9e@eb0eeae1-e81a-4a23-a4e4-ca8688e0a18a' is invalid.

This (eb0eeae1-e81a-4a23-a4e4-ca8688e0a18a) is my Tenant Id but what is this (5f1fb9dd-2fc2-4f4d-8fa4-ebd144432b9e).

I am sending to a default user (AlexW@M365x270547.OnMicrosoft.com) which is already there in my tenant.

Please help , am I missing anything?