Legal Question in Landlord & Tenant Law in California

Per the Code of Civil Procedure section 1013, the "Service is complete upon deposit in the mail." This is what's known the codification of the old English common law concept "Mailbox Rule".

Unfortunately Mr. Tashjian gave you a very simplistic answer to a more complex issue. To start with he only quoted part off the code section. The full text can be found here: https://leginfo.legislature.ca.gov/faces/codes_displaySection.xhtml?lawCode=CCP§ionNum=1013.

He also fails to note that the code section only applies to “civil actions,” meaning lawsuits. If you are not responding to papers sent to you or being sent to you in a lawsuit, the section doesn’t even apply.

Outside of lawsuits there are all kinds of other laws and rules about whether postmark or actual delivery date is what matters. Without knowing what you were responding to, and what your response was, there is no way to correctly answer your question.