Tell members when something is back in stock
Turn it on under Settings → Add-ons → Back-in-Stock Waitlist → See more. When an item is sold out, members see a small “email me when it’s back” box on its page. You can see who is waiting on the product’s own Inventory tab.
The ordinary way it works. You set the item back in stock from its own edit screen, everyone waiting gets an email, and the list clears. You get a short note saying how many were told.
When stock moves in bulk, the emails are held back on purpose. If stock changes through an import, a bulk edit on the Products screen, or a reconciliation that touches hundreds of items at once, nobody is emailed. Instead the restock is parked, and a notice appears at the top of your admin screens saying how many members are waiting on how many items, and why they were held.
This is deliberate. A single stock reconciliation can move hundreds of products in one go; without this, every waiting member on every one of those items would get a mail in the same minute. Held is recoverable. A blast is not.
To send a parked batch, open that item’s own edit screen and set it out of stock and back in. That is a single, deliberate change, so the emails go.
Nobody is ever told twice. If a send is interrupted halfway — a slow mail server, a request that times out — the members already emailed are remembered, and only the ones who were missed get the mail when it runs again.