Currently, we check the cache only immediately before attempting to send
an email address. However, we only reach that point if the rate limiter
(used to respect the daily API quota) permits it. As a result, around
40% of sends are wasted on email addresses that are ultimately skipped
due to cache hits.
Replace the pre-send cache `Seen` check with an atomic `StoreIfAbsent`
executed before the `limiter.Wait()` so that limiter tokens are consumed
only for email addresses that actually need sending. Skip the
`limiter.Wait()` on cache hits, remove cache entries only when a send
fails, and increment metrics only on successful sends.
Samantha Frank