contact/Services/MailSender.cs

using System.Threading.Tasks;
using MailKit.Net.Smtp;
using MailKit.Security;
using Microsoft.Extensions.Configuration;
using MimeKit;

namespace contact.Services
{
    //Sends web form submissions to support.
    //From is always the authenticated account; the visitor goes in Reply-To, so there is no sender spoofing.
    //SMTP settings come from the "Smtp" config section; the password is never in source.
    public class MailSender
    {
        private const string SupportAddress = "support@ayanova.com";
        private readonly IConfiguration _configuration;

        public MailSender(IConfiguration configuration)
        {
            _configuration = configuration;
        }

        public async Task SendToSupportAsync(string subject, string body, string visitorName, string visitorEmail)
        {
            var smtp = _configuration.GetSection("Smtp");

            var message = new MimeMessage();
            message.From.Add(new MailboxAddress("AyaNova web forms", smtp["User"]));
            message.To.Add(MailboxAddress.Parse(SupportAddress));
            if (MailboxAddress.TryParse(visitorEmail, out var replyTo))
            {
                replyTo.Name = visitorName;
                message.ReplyTo.Add(replyTo);
            }
            message.Subject = OneLine(subject);
            message.Body = new TextPart("plain") { Text = body };

            using var client = new SmtpClient();
            await client.ConnectAsync(smtp["Host"], smtp.GetValue<int>("Port"), SecureSocketOptions.StartTls);
            await client.AuthenticateAsync(smtp["User"], smtp["Password"]);
            await client.SendAsync(message);
            await client.DisconnectAsync(true);
        }

        //Subjects include visitor-supplied text; a hand-crafted POST could put line breaks in it
        private static string OneLine(string s) => s.Replace("\r", " ").Replace("\n", " ");
    }
}