By default, Android limits apps to only sending 30 SMS messages per 30 minutes (on newer versions of Android starting with 4.1 Jelly Bean), or 100 SMS messages per hour (on older versions of Android). If you send multi-part SMS messages, Android counts each part of the SMS message separately, resulting in an even lower maximum number of messages sent per hour.
If an app goes over this limit, Android will stop the app from sending messages until the user confirms that they really want to send more messages.
However, the Android phone itself is able to send SMS messages at a much higher rate, typically between 1000 and 1500 messages per hour depending on the phone and the mobile network.
Telerivet keeps track of its rate of sending SMS messages in order to avoid exceeding Android's per-app rate limit. (Occasionally, Telerivet's count may become incorrect, which can cause Android to display a prompt like "A large number of SMS is being sent. Click OK to continue or cancel to stop".)
To avoid this limit, the easiest way is to install SMS Expansion Packs. There are 29 SMS Expansion Packs, each allowing Telerivet to send an additional 100 SMS messages per hour (on older versions of Android) or 30 SMS messages per 30 minutes (on newer versions of Android).
To install SMS Expansion Packs, open the Telerivet Android app, then click Increase SMS Rate Limit.
On newer versions of Android, installing all expansion packs installed will allow the Telerivet Android app to send up to 1800 SMS messages per hour, subject to limitations of your Android phone's hardware and the mobile network.
On older versions of Android, it is only necessary to install 17 expansion packs in order to send up to 1800 SMS messages per hour.
Only the first 9 SMS Expansion Packs are available in Google Play. Additional SMS Expansion Packs are available for direct download (in APK format) by clicking More SMS Expansion Packs.
Note that Android's sending rate limit is unrelated to the maximum limit of messages per day in your Telerivet service plan, so adding SMS Expansion Packs is necessary even if you are on a paid service plan. SMS Expansion Packs are free to install, and are necessary to avoid a technical limitation on the Android platform.