To help prevent abuse of accounts, whether intentional or by mistake, we impose an hourly message and bandwidth limit on all accounts. This hourly limit is calculated as a percentage of your pre-paid or free message quota available within your package. For example, by default we currently allow all customers to use 10% of their entirely monthly quota in an hour i.e. you can spike to 73x your average message rate for the month in just one hour.
For the exhaustive and up to date list of all standard limits on accounts, please see the connection, channel, message and rate limits article.