[roybean]

The statute to start with is Cal Code Civ Pro 415.20. The case to start with is Ellard v. Conway (2001) 94 Cal.App.4th 540, 545-547, 114 Cal.Rptr.2d 399, 402-403.

In Ellard, substitute service was proper where a process server first attempted to personally serve defendants at their last known residential address, and, upon learning they no longer lived there, obtained a forwarding address from the U. S. Postal Service.

This amounted to reasonable diligence in attempting personal service thus making substitute service available.

Substitute service at a private/commercial post office box was proper, because that was the forwarding address provided to the Postal Service, thus making it defendants' "usual mailing address,"

Also, the manager of the facility knew the defendants and told the process server they received mail there.

Under the circumstances, it was more likely than not the manager would deliver the summons and complaint, and no facts suggested personal or substituted service was available at any other address or on any other individual.

Thus, CCP § 415.20(b) authorized substitute service on defendants at their private post office box.

Many cases have followed Ellard, most are unpublished California cases.
There is a good federal case that followed Ellard. I say good because Ellard itself actually distinguished a federal case that went the other way.
The case is Smilde v. Melchner, No. C-00-2687 JCS (N.D. Cal. Feb. 15, 2002).
2002 U.S. Dist. LEXIS 3809, and it discusses the matter at some length.