Why can't Apple's mobile phone im messages be sent out?

Apple's mobile phone im messages can't be sent all the time because the messages sent are considered as spam messages or harassing messages by the Apple system, so the system restricts the account. You can wait a while before resending, or avoid frequent sending.

The reasons why Apple's mobile phone can't send short messages are arrears, poor cell phone signal, loose or damaged sim card, system failure and so on.