Отправка email в ASP.NET Core через SMTP

dotnet add package MailKit

{
  "EmailSettings": {
    "Host": "smtp.fmailer.ru",
    "Port": 587,
    "UseStartTls": true,
    "SenderEmail": "noreply@mail.example.ru",
    "SenderName": "Магазин"
  }
}

dotnet user-secrets set "EmailSettings:Username" "<логин токена>"
dotnet user-secrets set "EmailSettings:Password" "<пароль токена>"

using MailKit.Net.Smtp;
using MailKit.Security;
using Microsoft.Extensions.Options;
using MimeKit;

public class EmailService
{
    private readonly EmailSettings _settings;

    public EmailService(IOptions<EmailSettings> settings) => _settings = settings.Value;

    public async Task SendAsync(
        string to,
        string subject,
        string html,
        string text,
        CancellationToken ct = default)
    {
        var message = new MimeMessage();
        message.From.Add(new MailboxAddress(_settings.SenderName, _settings.SenderEmail));
        message.To.Add(MailboxAddress.Parse(to));
        message.Subject = subject;

        // Обе части, чтобы письмо было multipart/alternative, а не только HTML.
        message.Body = new BodyBuilder { HtmlBody = html, TextBody = text }.ToMessageBody();

        using var client = new SmtpClient { Timeout = 10_000 };

        var option = _settings.UseStartTls
            ? SecureSocketOptions.StartTls        // порт 587
            : SecureSocketOptions.SslOnConnect;   // порт 465

        await client.ConnectAsync(_settings.Host, _settings.Port, option, ct);
        await client.AuthenticateAsync(_settings.Username, _settings.Password, ct);
        await client.SendAsync(message, ct);
        await client.DisconnectAsync(true, ct);
    }
}

builder.Services.Configure<EmailSettings>(
    builder.Configuration.GetSection("EmailSettings"));

builder.Services.AddTransient<EmailService>();

[HttpPost]
public async Task<IActionResult> Create(OrderRequest request, CancellationToken ct)
{
    var order = await _orders.CreateAsync(request, ct);

    await _email.SendAsync(
        to: order.CustomerEmail,
        subject: $"Подтверждение заказа №{order.Id}",
        html: "<h1>Заказ принят</h1><p>Доставка — завтра до 18:00.</p>",
        text: "Заказ принят. Доставка — завтра до 18:00.",
        ct);

    return Ok(order);
}

var builder = new BodyBuilder
{
    HtmlBody = "<p>Счёт за март во вложении.</p>",
    TextBody = "Счёт за март во вложении.",
};

await builder.Attachments.AddAsync("/srv/invoices/2026-03.pdf", ct);
builder.Attachments.Add("invoice.pdf", pdfBytes, new ContentType("application", "pdf"));

// Картинка в теле письма — в HTML как <img src="cid:logo">
var logo = builder.LinkedResources.Add("logo.png", logoBytes);
logo.ContentId = "logo";

message.Body = builder.ToMessageBody();

// Кладём письмо в канал, чтобы HTTP-ответ не ждал SMTP.
public class EmailBackgroundService : BackgroundService
{
    protected override async Task ExecuteAsync(CancellationToken ct)
    {
        await foreach (var job in _queue.ReadAllAsync(ct))
        {
            try { await _email.SendAsync(job.To, job.Subject, job.Html, job.Text, ct); }
            catch (Exception ex) { _logger.LogError(ex, "Письмо на {To} не отправлено", job.To); }
        }
    }
}