//POST: api/Email
    [HttpPost]
    public async Task<IActionResult> Post()
    {
        List<Attachment> attachments;
        mailMessage.Attachments.Add(new Attachment(@"C:a.pdf"));
        mailMessage.Attachments.Add(new Attachment(@"C:b.pdf"));

        using (MailMessage mailMessage = new MailMessage())
        {
            mailMessage.To.Add("someone@somehwere.com");

            using (var smtpClient = new SmtpClient("smtp.gmail.com", 578))
            {
                smtpClient.Credentials = new NetworkCredential("me@gmail.com", "12345");
                smtpClient.EnableSsl = true;
                await smtpClient.SendMailAsync(mailMessage);
            }
        }

        File.Delete(@"C:a.pdf");
        File.Delete(@"C:b.pdf");

        return Ok();
    }

foreach (var attachment in mailMessage.Attachments)
    attachment.Dispose();