Avelto
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Send email from .NET

Five steps: create a project, create a test key, send to the sandbox, read the log, then verify a domain so you can send to anyone.

1. Install

No SDK needed. The API is plain JSON over HTTPS, so HttpClient and System.Text.Json from the base class library are all it takes. Start a console project (.NET 8 or later):

shell
dotnet new console -n AveltoQuickstart && cd AveltoQuickstart

2. Create an API key

Sign in, open API keys in the dashboard and create a test key. It starts with av_test_. Export it so the program can read it (on PowerShell, $env:AVELTO_API_KEY = "av_test_..."):

shell
export AVELTO_API_KEY=av_test_...
Sandbox rules

Test keys never deliver anything; they run the pipeline and record events. The sandbox sender you@sandbox.avelto.dev only delivers to your account's verified owner email and to the simulator addresses delivered@, bounced@ and complained@sandbox.avelto.dev. Anything else is refused with 403 sandbox_recipient_not_allowed. To send to anyone, verify a domain (step 5).

3. Send your first email

Every request carries the key as a bearer token. A rejected send comes back as a non-2xx status with a JSON body of { "error": { "code", "message" } }.

C#
// Program.cs
using System.Net.Http.Headers;
using System.Net.Http.Json;
using System.Text.Json;

var http = new HttpClient { BaseAddress = new Uri("https://api.avelto.dev") };
http.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue(
    "Bearer", Environment.GetEnvironmentVariable("AVELTO_API_KEY"));

var response = await http.PostAsJsonAsync("/v1/emails", new Dictionary<string, string>
{
    ["from"] = "[email protected]",
    ["to"] = "[email protected]",
    ["subject"] = "Hello from Avelto",
    ["text"] = "It works.",
});

var json = await response.Content.ReadFromJsonAsync<JsonElement>();
if (!response.IsSuccessStatusCode)
{
    var error = json.GetProperty("error");
    Console.Error.WriteLine($"{(int)response.StatusCode} " +
        $"{error.GetProperty("code")}: {error.GetProperty("message")}");
    return;
}

var id = json.GetProperty("id").GetString();
Console.WriteLine(id); // "9c1f4a52-6f6e-4b8f-9b8e-2e1a5c7d3f10"
shell
dotnet run

The API answers 201 Created with the email id:

HTTP
HTTP/1.1 201 Created
Content-Type: application/json

{ "id": "9c1f4a52-6f6e-4b8f-9b8e-2e1a5c7d3f10" }
Retrying safely

Send an Idempotency-Key header (any unique string, such as your order id) with every POST /v1/emails. If the request times out or comes back 429, 502, 503 or 504, wait a moment and send it again unchanged with the same key: the API returns the original email id instead of sending twice. See Idempotency.

4. Check the log

Fetch the email by id. status moves from queued to sent to delivered, and events records each step: email.queued, email.sent, email.delivered.

C#
var email = await http.GetFromJsonAsync<JsonElement>($"/v1/emails/{id}");

// "queued", then "sent", then "delivered"
Console.WriteLine(email.GetProperty("status").GetString());
foreach (var e in email.GetProperty("events").EnumerateArray())
{
    Console.WriteLine($"{e.GetProperty("type")} {e.GetProperty("occurred_at")}");
}
JSON
{
  "id": "9c1f4a52-6f6e-4b8f-9b8e-2e1a5c7d3f10",
  "mode": "test",
  "from": "[email protected]",
  "to": ["[email protected]"],
  "subject": "Hello from Avelto",
  "status": "delivered",
  "events": [
    {
      "id": "e1f0c3a4-8b2d-4c6e-9a1f-5d7b3e2c8a90",
      "type": "email.queued",
      "payload": {},
      "occurred_at": "2026-09-17T10:12:04.000Z"
    },
    {
      "id": "a7c2e9d1-3f4b-4a8e-b6c0-2d9e1f7b5c34",
      "type": "email.sent",
      "payload": { "test": true, "ses_message_id": "test-9c1f4a52-6f6e-4b8f-9b8e-2e1a5c7d3f10" },
      "occurred_at": "2026-09-17T10:12:06.000Z"
    },
    {
      "id": "c4b8d2f6-7e1a-4d3c-8f9b-6a2e0c5d1b78",
      "type": "email.delivered",
      "payload": { "test": true, "recipients": ["[email protected]"] },
      "occurred_at": "2026-09-17T10:12:06.000Z"
    }
  ]
}

5. Verify a domain

Add a domain, publish the DNS records it prints (three DKIM CNAMEs, an SPF TXT and a DMARC TXT), then poll GET /v1/domains/:id until status is verified. The GET re-checks DNS on every call. Use a subdomain such as mail.acme.com.

C#
var created = await http.PostAsJsonAsync(
    "/v1/domains", new { name = "mail.acme.com" });
created.EnsureSuccessStatusCode();
var domain = await created.Content.ReadFromJsonAsync<JsonElement>();

foreach (var r in domain.GetProperty("dns_records").EnumerateArray())
{
    Console.WriteLine($"{r.GetProperty("type")}\t{r.GetProperty("name")}\t" +
        $"{r.GetProperty("value")}\t({r.GetProperty("purpose")})");
}

// Publish the records, then poll. GET re-checks DNS on every call.
var domainId = domain.GetProperty("id").GetString();
var status = domain.GetProperty("status").GetString();
while (status == "pending")
{
    await Task.Delay(TimeSpan.FromSeconds(30));
    var current = await http.GetFromJsonAsync<JsonElement>($"/v1/domains/{domainId}");
    status = current.GetProperty("status").GetString();
}
Console.WriteLine(status); // "verified" or "failed"

Once the domain is verified, switch AVELTO_API_KEY to a live key (av_live_) and change from to an address on it, such as [email protected]. Nothing else changes.

Next

  • Send email: every field, attachments, tags, scheduling and idempotency.
  • Webhooks: get events pushed to your app.
  • Test mode: test keys, the sandbox sender and the simulator addresses.