Install

dotnet add package OpenEmail

using OpenEmail; var client = new OpenEmailClient(); var sent = await client.Emails.SendAsync(new Body{    ["from"] = "Acme Billing <[email protected]>",    ["to"] = "[email protected]",    ["subject"] = "Your September invoice",    ["html"] = "<p>Invoice attached.</p>",}); Console.WriteLine($"{sent["id"]} {sent["status"]}");

new OpenEmailClient() reads the key from OPENEMAIL_API_KEY, so the code holds no credential. Pass new OpenEmailOptions { ApiKey = ... } when your configuration lives somewhere else. A missing or malformed key throws ArgumentException when the client is built, before anything is sent.

A request body is a Body, or any IReadOnlyDictionary<string, object?>, whose keys keep the names of the API, so scheduledAt is written the same way here as in the API reference. A response is a JsonObject from System.Text.Json.Nodes, read with sent["id"].

On .NET 10 one file runs without a project. The #:package line fetches the package, and dotnet run send.cs sends the message.

#:package OpenEmail@* using OpenEmail; var client = new OpenEmailClient(); var sent = await client.Emails.SendAsync(new Body{    ["from"] = "[email protected]",    ["to"] = "[email protected]",    ["subject"] = "Sent from one file",    ["text"] = "No project was needed.",}); Console.WriteLine(sent["id"]);

The package targets net8.0 and netstandard2.0. On .NET 8 and later it has no dependencies, and it is safe to trim and to compile ahead of time. On .NET Standard 2.0 it brings System.Text.Json and Microsoft.Bcl.AsyncInterfaces. A client is safe to use from many threads at once: build one when the program starts and share it.