public class DiskSmtpClient : SmtpClient
    {
        public DiskSmtpClient(IOptionsSnapshot<MailKitOptions> mailOptionsSnapshot)
        {
            if (mailOptionsSnapshot.Value.SpecifiedPickupDirectory)
            {
                SpecifiedPickupDirectory = true;
                PickupDirectoryLocation = mailOptionsSnapshot.Value.PickupDirectoryLocation;
            }
            
        }
        public bool SpecifiedPickupDirectory { get; set; }
        public string PickupDirectoryLocation { get; set; }

        public override Task SendAsync(MimeMessage message, CancellationToken cancellationToken = new CancellationToken(),
            ITransferProgress progress = null)
        {
            if (!SpecifiedPickupDirectory)
                return base.SendAsync(message, cancellationToken, progress);
            return SaveToPickupDirectory(message, PickupDirectoryLocation);

        }

    



        private async Task SaveToPickupDirectory(MimeMessage message, string pickupDirectory)
        {
            using (var stream = new FileStream($@"{pickupDirectory}\email-{Guid.NewGuid().ToString("N")}.eml", FileMode.CreateNew))
            {
                await message.WriteToAsync(stream);
            }
        }

       

        public override Task ConnectAsync(string host, int port = 0, SecureSocketOptions options = SecureSocketOptions.Auto,
            CancellationToken cancellationToken = new CancellationToken())
        {
            if (!SpecifiedPickupDirectory)
                return base.ConnectAsync(host, port, options, cancellationToken);
            return Task.CompletedTask;
        }



        public override Task DisconnectAsync(bool quit, CancellationToken cancellationToken = new CancellationToken())
        {
            if (!SpecifiedPickupDirectory)
                return base.DisconnectAsync(quit, cancellationToken);

            return Task.CompletedTask;
        }
    }

 services.AddTransient<DiskSmtpClient>();

  var email = "mail@dotnettips.info";
            var subject = "subject";
            var message = "message";

            var emailMessage = new MimeMessage();

            emailMessage.From.Add(new MailboxAddress("DNT", "do-not-reply@dotnettips.info"));
            emailMessage.To.Add(new MailboxAddress("", email));
            emailMessage.Subject = subject;
            emailMessage.Body = new TextPart(TextFormat.Html)
            {
                Text = message
            };

            
                _client.SpecifiedPickupDirectory = true;
                _client.PickupDirectoryLocation = "c:\\mail";

                _client.LocalDomain = "dotnettips.info";
                await _client.ConnectAsync("smtp.relay.uri", 25, SecureSocketOptions.None).ConfigureAwait(false);
                await _client.SendAsync(emailMessage).ConfigureAwait(false);
                await _client.DisconnectAsync(true).ConfigureAwait(false);

services.Configure<MailKitOptions>(options => Configuration.GetSection("MailKitOptions").Bind(options));

  "MailKitOptions": {
    "SpecifiedPickupDirectory": true,
    "PickupDirectoryLocation": "c:\\mail"
  }

    public class MailKitOptions
    {
        public  bool SpecifiedPickupDirectory { get; set; }
        public  string PickupDirectoryLocation { get; set; }
    }

An unhandled exception has occurred while executing the request. 
MailKit.Security.SslHandshakeException: An error occurred while attempting to establish an SSL or TLS connection. 
The host name did not match the name given in the server's SSL certificate.