Country rules on sending SMS messages

Regulations typically pertain to the following:

Time of day that messages may be delivered
Content of the messages
Format of the Sender ID (from address) that may be used/displayed. Carriers in certain countries will block SMS messages that are sent from phone number formats that they do not recognize or support. To help ensure that your messages are delivered, the from address of a message may be replaced with a format that will be accepted by the carrier of the mobile device.

Clickatell has some great guidance on specific regulations for the following countries: