You will see this AntiSpam message if you try to send too many in-game messages in too short a time span. For example, if you send 10 messages in just a few minutes, you will see the AntiSpam warning. You will need to wait 10 minutes before sending another one message. If you want to message everyone in your alliance, you should instead simply type [ally] in the recipient section. You do need special permission set on your account by the alliance owner before you can use this feature.