1,
According to the provisions of Article 77 of the Civil Procedure Law of People's Republic of China (PRC), the service of litigation documents must have a receipt, and the addressee should record the date of receipt on the receipt and sign or seal it.
The date of receipt by the addressee on the service receipt shall be the date of service.
Article 78 Litigation documents shall be served directly on the addressee. If the addressee is a citizen, I will not give it to his adult family to sign for it; If the addressee is a legal person or other organization, it shall be signed by the legal representative of the legal person, the principal responsible person of other organizations or the responsible person of the legal person or organization; If the addressee has an agent ad litem, he may send it to his agent for signature; If the addressee appoints an agent to the people's court, it shall send it to the agent for signature.
The date of receipt by the adult family members of the addressee, the legal person or other organization responsible for receiving the goods, litigation agent or agent shall be the date of delivery.
Therefore, the service must be delivered to the recipient himself, otherwise the procedure is illegal. In the case of delivery, it is obviously wrong for you to sign on your behalf.
2,
Article 79 If the addressee or an adult family member living together refuses to accept the litigation documents, the addressee shall invite the representatives of the relevant grass-roots organizations or units to be present, explain the situation, record the reasons and date of refusal on the service receipt, and leave the litigation documents at the addressee's residence with the signatures or seals of the addressee and witnesses, which shall be deemed as service.
Article 80 If it is difficult to serve litigation documents directly, it may entrust other people's courts to serve them on its behalf or by post. If it is delivered by post, the date of receipt indicated on the receipt shall be the date of delivery.
For example, in this case, you signed it, and Zhang San's mother said she disagreed, but it did not hinder the effectiveness of the service. In other words, even if the other party really disagrees, as long as the delivery meets the conditions, it is a legal and effective delivery behavior. However, in this case, due to the lack of effective delivery procedures, this delivery is invalid. It's unreasonable for Zhang San to blame you, because it's not up to them to answer the lawsuit or not. If it is legal and valid, the delivery conditions will take effect.
It is necessary to bear certain responsibilities on behalf of the signing. As for what this responsibility is, there is no legal provision. But with your signature, the Zhang family can use this as an excuse to say that the procedure is illegal and not to participate in the lawsuit. Don't be afraid, there won't be a big problem.