For every customer whose birthday is today and who hasn't already been credited this year, Sprwt stamps the current year on their record (so they can't be credited twice in the same year) and always sends the automated birthday email. Cashback is only added to their store balance if the Marketing Hub module is active and your Birthday Present Amount (Client Config, tab Marketing: Hub) is greater than $0 — the birthday email itself still goes out either way. Setting Birthday Present Amount to exactly -1 skips the whole feature for everyone (no emails, no credits).