protected void Send( )
    {
        var messages = new List<MimeMessage>( );

        for( int i = 1; i < 4; i++ )
        {
            var message = new MimeMessage
            {
                Sender = new MailboxAddress( "Sender Name", "Sender Email Address" ),
                Subject = "Test email " + i.ToString( ),
                Body = new TextPart( TextFormat.Html ) { Text = "Test body " + i.ToString( ) }
            };
            message.To.Add( new MailboxAddress( "test" + i.ToString( ) + "@example.com" ) );
            messages.Add( message );
        }

        MailNew.SendMultipleAsyncMails( messages );
    }

public static void SendMultipleAsyncMails( List<MimeMessage> messages )
    {
        Task t = Task.Run( async ( ) =>
        {
            using( var client = new SmtpClient( ) )
            {
                await client.ConnectAsync( "Host", 587, SecureSocketOptions.StartTls );
                await client.AuthenticateAsync( "UserName", "Password" );
                foreach( var message in messages )
                {
                    await client.SendAsync( message );
                    await Task.Delay( 10000 );
                }
                await client.DisconnectAsync( true );
            }
        } );
    }