Bob is required to serve a pre-court proceedings’ notice [or complaint/other filing even] on Cob. Bob and Cob are both [U.S./California] citizens.
Bob sends all documents required to Cob in a regular email or letter neither of which constitute proof of service. However, Cob replies (1) addressing the letter by its title, (2) and expressly acknowledges receipt, in fact, acts upon the notice or even engages in continued exchange with Bob wherein Cob accepts liability for the subject matter Bob needed to serve the documents on Cob.
Is there any case law that would support that Cob was served upon the notice and his reply, acknowledgement, replies and/or acknowledgement of his liability which required the service constitute proof of service of process?
Is there any U.S. case for similar circumstances where a direct explicit reply to such letter or email containing all information to be served on another party would constitute proof of service of process if the reply comes from the party to be served?
My research so far...
The only law that explicitly considers replies as proof of services seems to be from South Australia:
"Email service in accordance with rule 42.2.
This means sending the documents as an attachment in PDF or Word format to an address used by the recipient and the recipient replying or actively acknowledging receipt of the email. However, a response generated automatically by the recipient’s email service is not considered a reply or acknowledgment of service [r 42.2 (3)]." Not unreasonable, is it?
Are there any exceptions per case law? Excused by the other parties conduct, etc.?