There is a built-in daily limit check that limits a certain number of messages to be sent per day. The daily limit is designed to keep your account in good standing with Amazon's Terms of Service. If you exceed the daily limit, Amazon can block your Seller account from sending messages altogether.
How is the daily limit calculated?
To calculate your daily limit, we multiply your average daily order volume over the last 7 days by 4. Then, we add an additional 600 messages to this amount.
The formula is: 4 * ave. daily orders over the last 7 days + 600
For example, if you have 1,024 orders from the past 7 days then your daily limit would be 1185 messages per day. e.g. 4 x 146 + 600 = 1185.14