As oddly as it may sound, there is currently no mail service in Guatemala. As I understand, it was kind of privatized for a while, but the contract expired, and it was not renewed, So if you need mail to Guatemala, you will need to find some alternatives.

Guatemalan postal codes are the equivalent of Zip Codes, they are assigned by zone in Guatemala City, and mostly by Department and Municipality in the rest of the country. The official site of the Guatemala Mail Office is here, the service is private, at works well and it is handled by the same company that handles some private mail companies, including Cargo Expreso and UPS.