Amazon limits how many messages your account can send in a day. The official limit is 5 times your average daily orders (calculated from the past 7 days) + 500.
In order to ensure that you can still send your own emails to customers queries, FeedbackExpress is limited to sending the following,
(Number of orders for the past 7 days * 4) / 7 + 500
Example: You have 1 channel and had 700 orders for the past 7 days, then you will be able to send 900 messages per day (700 * 4) / 7 + 500 = 900 messages.
If you need to send more messages than this on any given day FeedbackExpress will queue your messages to go over several days. All pending emails can be seen by going to Campaigns and clicking on the Pending tab. You may notice an alert on your dashboard warning you of this: