There are special rules when serving someone outside of the United States. Service may be by any internationally agreed means reasonably calculated to give notice, such as authorized by the Hague Convention on the Service Abroad of Judicial and Extrajudicial Documents.
The most convenient place to research service in a foreign country is Travel.State.Gov. The State Department's web site contains information concerning most countries (such as service in Mexico at http://travel.state.gov/law/judicial/judicial/judicial_677.html).
To research any country go to The State Department's web site at http://travel.state.gov and in the search box type in service of process and the name of the country, for example "service of process japan".
Your printed documents will contain additional instructions for service abroad if you select the option for service outside of the United States.
NOTE: You must comply with the law of the country where you are serving. If the country authorizes service in the manner set out in the Hague Convention you must comply by obtaining form USM-94, translating the SUMMONS and VERIFIED PETITION FOR DIVORCE into the language used in the country where yiou are serving, and having the Central Authority for that country serve your SUMMONS and VERIFIED PETITION FOR DIVORCE.
Click here to obtain form USM-94 from the United States Marshall's Office