List<string> addresses = GetEmailAddresses();
var skipCount = 0;
const int takeCount = 50;
TimeSpan period = TimeSpan.FromMinutes(30);

var timer = new System.Threading.Timer(e =>
{
    foreach (string address in addresses.Skip(skipCount).Take(takeCount))
    {
        SendEmailAddress(address);
    }

    skipCount += takeCount;
}, null, TimeSpan.Zero, period);

while (addresses.Count > skipCount) { }

timer.Dispose();

while (addresses.Count > skipCount) { }

Queue<string> addresses = GetEmailAddresses();
const int takeCount = 50;
TimeSpan period = TimeSpan.FromMinutes(30);

while (addresses.Any()) {
  string address = addresses.Dequeue();
  await SendEmail(address);
  if (addresses.Count % takeCount == 0) {
    await Task.Delay(period);
  }
}