A replenishment email is a reminder that can be sent to customers who ordered products which would normally need to be re-ordered after a certain period of time.
For example, let's say you perfume which would normally run out after 30 to 60 days.
This trigger is one of the only ones that requires a bit of configuration on the merchant's end. Unfortunately there's no way to determine the replenishment period for a given product without that information being provided.
So what you can do is create an attribute called `magemail_days_to_replenish` and populate it for all of the products that you would like to. Note that it should have a type of Text Field with validation of Integer Number.
Then the replenishment trigger will be able to send out reminders based on those products having been purchased by customers.
You can also use another attribute name if you happened to already have an attribute by a different name.