Legal Question in Credit and Debt Law in California

No, not if a copy was not also mailed to the same address. Bear in mind, though, that lawsuits don't get tossed out due to defective service. It only delays things. And too often the plaintiff or people working with them will file proof of service that are not correct. In which case the court will move the case forward as if service has been properly completed and you may wind up having to argue the defective service as grounds to set aside a default judgment against you.

I like to analogize it to driving. Say you're approaching an intersection. The light is green for you, but you see a huge semi barreling down on the intersection and not slowing down. Sure, you have the right of way, but do you really want to assert that right and make a turn in front of it? Or do you just want to wait and turn in behind it and follow along?

So the best thing to do is contact the plaintiff's attorney, or plaintiff if in pro per, notify them of the defective service and negotiate either re-service or a reasonable time to respond in exchange for not wasting everyone's time with a motion to quash service that can only result in you being served again, properly, anyway.