异步发送邮件不起作用

由于您的"fire-and-forget"，您将立即处理邮件消息；这可能会影响到你。此外，您没有处理邮件客户端。您应该将整个操作放入"fire-and-forget"中，包括SmtpClient的创建和处理。更像这样的东西：

FireAndForgetTask(async cancellationToken =>
{
                using(var smtp = new SmtpClient
                {
                    Host = "myhost",
                    Port = 587,
                    EnableSsl = false,
                    DeliveryMethod = SmtpDeliveryMethod.Network,
                    Credentials = new NetworkCredential("myemail", "mypass"),
                    Timeout = 50000
                })
                {          
                    using (var message = new MailMessage("myemail", destMail)
                    {
                        Subject = subject,
                        Body = mailBody,
                        IsBodyHtml = html           
                    })
                    {
                       await smtp.SendMailAsync(message);
                    }
                }
}