Send email in C# / .NET via HTTP API (no SMTP)

Send email from C# with one HTTP call — no SmtpClient, no MailKit. The send code with HttpClient, how to receive the reply, and error handling.

Sending email from C# used to mean SmtpClient — which Microsoft now discourages — or MailKit plus SMTP config. A free HTTP email API replaces both with one HttpClient call, and it can read the reply, which an SMTP sender can’t.

Send email in C#

HttpClient with System.Net.Http.Json makes it a couple of lines:

using System.Net.Http.Json;

var http = new HttpClient { BaseAddress = new Uri("https://login.ollastack.com") };
http.DefaultRequestHeaders.Authorization = new("Bearer", "fmd_…");

// create a mailbox once
var created = await http.PostAsJsonAsync("/api/mailboxes",
    new { name = "dotnet", mode = "agent", handle = "net" });
created.EnsureSuccessStatusCode();
var mbx = await created.Content.ReadFromJsonAsync<Dictionary<string, object>>();
var id = mbx!["id"];

// send
var res = await http.PostAsJsonAsync($"/api/mailboxes/{id}/send",
    new { to = "user@example.com", subject = "Hi", text = "Sent from .NET." });
res.EnsureSuccessStatusCode();

No SmtpClient, no SMTP host or port — one HTTPS request that returns a msg_… id.

Receive the reply

The same HttpClient long-polls for the next inbound message:

var msg = await http.GetFromJsonAsync<Dictionary<string, object>>(
    $"/api/mailboxes/{id}/wait?timeout=60");
Console.WriteLine(msg!["subject"]); // codes[] holds the extracted OTP

That’s the half SmtpClient can’t do — read inbound mail and pull the code without parsing it yourself.

Send HTML, not just text

Swap text for html (or send both — text is the fallback for clients that don’t render HTML):

await http.PostAsJsonAsync($"/api/mailboxes/{id}/send", new {
    to      = "user@example.com",
    subject = "Welcome",
    html    = "<h1>Welcome</h1><p>Thanks for signing up.</p>",
    text    = "Welcome — thanks for signing up.",
});

The same endpoint takes both; add a reply_to to route replies to a specific address.

Why an API beats SmtpClient / MailKit

SmtpClient is discouraged for new code and only sends; MailKit still needs SMTP config. The HttpClient call sends from an authenticated domain, returns a structured error, and also receives — handy for OTP testing or agent inboxes.

Error handling

EnsureSuccessStatusCode() throws on failure; for finer control, check res.StatusCode and read the JSON error body for the code and message, backing off on 429. Register the HttpClient with IHttpClientFactory in DI rather than newing one per call, so connections are pooled and the base address and auth header are configured once.

See the email API overview, the Java version, and the Kotlin version.

Get a free token — send and receive from .NET, no card.

Last updated June 21, 2026. Spotted something out of date? Email hello@ollastack.com.