How can I create replenishment data within Magento?

If you already have the magemail_days_to_replenish, you'll see this:

If not, you'll need to create it. Once it's created, here's how you'll enable it: Go into Catalog > Attributes > Manage Attribute Sets. Choose the attributes that contain products that replenish.

Select the attribute you wish to edit. Go under Unassigned Attributes and move magemail_days_to_replenish to Groups

Now, save and go to your product information. There will now be a field to enter replenishment data! Enter this information in days and MageMail will pick this up, automatically.

Go hog wild!