Amazon is calculating your daily limit by the number of orders you have, comparing your sent emails, to avoid spamming amazon customers.
Your daily limit calculation will be :
[[5 times daily order volume over the last 180 days/180] x 4] = [Daily limit]
if the [Daily limit] is lower then 250 emails, it will be set to 250, this limitation will help you to avoid sending to many emails out, so amazon won’t block you from sending mails.
you can find your daily limitation on your Auto-Responder dashboard.