wrapper

Wrapping it up in an Email Service
To encapsulate the email sending functionality and make it easy to send email from anywhere in your ASP.NET Core application you can create an EmailService class and IEmailService interface like below.

For examples of how this email service is used in a real project see ASP.NET Core 3.1 - Boilerplate API with Email Sign Up, Verification, Authentication & Forgot Password.

using MailKit.Net.Smtp;
using MailKit.Security;
using Microsoft.Extensions.Options;
using MimeKit;
using MimeKit.Text;
using WebApi.Helpers;

namespace WebApi.Services
{
    public interface IEmailService
    {
        void Send(string from, string to, string subject, string html);
    }

    public class EmailService : IEmailService
    {
        private readonly AppSettings _appSettings;

        public EmailService(IOptions<AppSettings> appSettings)
        {
            _appSettings = appSettings.Value;
        }

        public void Send(string from, string to, string subject, string html)
        {
            // create message
            var email = new MimeMessage();
            email.From.Add(MailboxAddress.Parse(from));
            email.To.Add(MailboxAddress.Parse(to));
            email.Subject = subject;
            email.Body = new TextPart(TextFormat.Html) { Text = html };

            // send email
            using var smtp = new SmtpClient();
            smtp.Connect(_appSettings.SmtpHost, _appSettings.SmtpPort, SecureSocketOptions.StartTls);
            smtp.Authenticate(_appSettings.SmtpUser, _appSettings.SmtpPass);
            smtp.Send(email);
            smtp.Disconnect(true);
        }
    }
}