EmailSender.cs

public interface IEmailSender
{
    void Send(string recipient, string subject, string htmlBody);
}

public class EmailSender : IEmailSender
{
    private readonly IEmailSettings _settings;

    public EmailSender(IEmailSettings settings)
    {
        _settings = settings;
    }

    public void Send(string recipient, string subject, string htmlBody)
    {
        var credentials = new NetworkCredential(_settings.Username, _settings.Password);

        using (var client = new SmtpClient(_settings.Host, _settings.Port) { Credentials = credentials })
        {
            client.Send(new MailMessage(_settings.Address, recipient, subject, htmlBody) { IsBodyHtml = true });
        }
    }
}

FileSystem.cs

public interface IFileSystem
{
    bool FileExists(string path);

    string GetFileNameWithoutExtension(string path);

    string ReadAllText(string path);
}

public class FileSystem : IFileSystem
{
    public bool FileExists(string path)
    {
        return File.Exists(path);
    }

    public string GetFileNameWithoutExtension(string path)
    {
        return Path.GetFileNameWithoutExtension(path);
    }

    public string ReadAllText(string path)
    {
        return File.ReadAllText(path);
    }
}

IEmailSettings.cs

public interface IEmailSettings
{
    string Host { get; }

    int Port { get; }

    string Address { get; }

    string Username { get; }

    string Password { get; }
}

TemplateTransformer.cs

public interface ITemplateTransformer
{
    string Transform<T>(string templatePath, T model);
}

public class TemplateTransformer : ITemplateTransformer
{
    private readonly IFileSystem _fileSystem;

    public TemplateTransformer(IFileSystem fileSystem)
    {
        _fileSystem = fileSystem;
    }

    public string Transform<T>(string templatePath, T model)
    {
        if (!_fileSystem.FileExists(templatePath))
        {
            throw new FileNotFoundException(string.Format("Could not find template '{0}'.", templatePath));
        }

        var templateName = _fileSystem.GetFileNameWithoutExtension(templatePath);

        var template = _fileSystem.ReadAllText(templatePath);

        using (var handlebars = new Handlebars())
        {
            handlebars.RegisterTemplate(templateName, template);

            return handlebars.Transform(templateName, model);
        }
    }
}