Note: This post may or may not be outdated, as it uses the SDK from when the Office 365 APIs were in Preview.
Because I couldn't find anything online, here's my contribution to the community. This code snippet shows you how to send a message using the C# SDK for the Exchange Mail REST API.
IMessage message = new Message(); Recipient recipient = new Recipient(); recipient.Name = "abcde"; recipient.Address = "email@example.com"; ItemBody body = new ItemBody(); body.Content = "Hey!"; message.Body = body; message.ToRecipients.Add(recipient); await mailClient.Me.Messages.AddMessageAsync(message);