When I call the endpoint https://graph.microsoft.com/v1.0/me/sendMail
using a Patch or Put method I receive a Method Not Allowed - 405
error.
Based on this https://learn.microsoft.com/en-us/graph/api/resources/message?view=graph-rest-1.0 documentation I get the expected behavior.
From my end I haven't been able to replicate the same behavior, of being able to update the email body of an an mail in my inbox.
Please share the request ID and the time stamp of when you made the API call, this would enable the team to investigate this case further.