How can I get 'fire and forget' behaviour from c# console code to web api

Check the below block of code. I need fire and forget behavior while calling sendMail method (i.e. while calling api). With this block of code I'm getting TaskCancelledException sometimes. Please help

private void ProcessMails()
{
  while(true)
  {

    var mailsToBeProcessed = getAllMailsToBeProcessed(alreadySent,      numOfMailsToBeProcessed);

    freach(var mail in Mails)
    {

        sendMail("mailsendingApiUrl", mail)

        alreadySent++;
    }

   Thread.Sleep(60000);
 }
}

private async void sendMail(string apiEndPoint, MailContent mailContent)
{
    using (var client = new HttpClient())
        {
             await client.PostAsJsonAsync(apiEndPoint, mailContent.ContentId);
        }
}