Hi @Namit Pandey ,
Hope you are doing well,
To set categories for message, you need to draft a message using this graph API endpoint. Refer
Post:-https://graph.microsoft.com/v1.0/me/messages
Once the message draft action is completed (above step) ,you can see this message in in your sent items with category selected. after this you have to send email by capturing the message ID from previous step.
To Send message Using this graph API endpoint https://graph.microsoft.com/v1.0/me/messages/{msgid}/send -Refer
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