"Although the defendants appeared in court, can they now claim the service was to someone unauthorized to accept on their behalf?" -- Yes. It appears that is just what they have done. Whether the judge will set aside their default depends on facts you have not stated. You need to oppose the defendants' motion with appropriate legal authority and facts if you do not want the defendants to prevail on the motion.
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You would probably find it helpful to contact a litigation attorney to review your situation. There are numerous facts that should be evaluated to determine the strength of any opposition motion to defendant's motion to set aside a "void" judgment.
Regarding service of process: some factors that would be considered are whether you are suing an individual or entity defendant. Whether you effected personal or substitute service. You need to explore the law dealing with authorized agents for service of summons.
Regarding general appearance: A general appearance by a party is equivalent to receiving service of summons. More facts are needed to know whether the defendant's appearance in court amounted to a general appearance and a recognition of the court's personal jurisdiction over the matter.
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You need to provide more facts in order to ascertain whether or not the defendants were properly sub-served. You also need to clarify what you mean by appear in court: just physical presence in court, or by filing an Answer? Did you effectuate the service or a third party? Did the server then mail a copy of the summons and complaint to the same address that the sub-service was made? If not, then the defendants can rightfully assert that they were not properly served.
Frank W. Chen has been licensed to practice law in California since 1988. The information presented here is general in nature and is not intended, nor should be construed, as legal advice for a particular case. This Avvo.com posting does not create any attorney-client relationship with the author. For specific advice about your particular situation, please consult with your own attorney.