public interface IInvioMail
{
    bool SendEmail(string destinatario, string _TestoEmail);
}

public class InvioMail:IInvioMail
{
    public bool SendEmail(string destinatario, string _TestoEmail)
    {
        try
        {
            MailMessage mail = new MailMessage();
            SmtpClient SmtpServer = new SmtpClient("smtp.gmail.com");

            mail.From = new MailAddress("your_email_address@gmail.com");
            mail.To.Add(destinatario);
            mail.Subject = "News Letter fanstastica";
            mail.Body = _TestoEmail;

            SmtpServer.Port = 587;
            SmtpServer.Credentials = new System.Net.NetworkCredential("username", "password");
            SmtpServer.EnableSsl = true;

            SmtpServer.Send(mail);
        }
        catch (Exception ex)
        {
            return false;
        }
        return true;

    }
}

public class InvioMailNewsLetter
{
    private LocalDbContext mLocalDbContext = new LocalDbContext();
    IInvioMail invioMail;
    public InvioMailNewsLetter(IInvioMail _invioMail)
    {
      invioMail = _invioMail;
    }

    public bool EseguiAnalisi(string _TestoEmail)
    {
        List<ListaMailNewsletter> ListaMailDaInviare = mLocalDbContext.SaldiFinali.Where(x => x.MailDaInviare && !string.IsNullOrEmpty(x.Mail)).ToList();

        foreach (var item in ListaMailDaInviare)
        {
            if (!invioMail.SendEmail(item.Mail, _TestoEmail))
                return false;

            item.MailDaInviare = false;

        }

        mLocalDbContext.SaveChanges();
        return true;
    }

}