See this post of Peter Bert on The Hague Service Convention, Default Judgments, and Deemed Service under German Law over at Letters Blogatory.
In a series of judgments on July 3 and July 17, 2012, the Federal Supreme Court (Bundesgerichtshof) has ruled on the compatibility of deemed service under German law with the Hague Service Convention. The Court held that only the first court document in a dispute must be served pursuant to the Hague Service Convention. Any subsequent service of court documents can be by post, in accordance with the provisions of domestic German law. Section 184 of the German Civil Code (ZPO), according to which “two weeks after it has been mailed, the document shall be deemed served,” applies to service of such documents. In the cases before the Federal Supreme Court, default judgments were served by post, and the time period for filing a protest (Einspruch) was determined on the basis of deemed service.
The rest of the post is here, including references to US cases and opinions on the issue.