If a defendant has been served with a complaint but has not appeared in the action, can discovery be served by mail or overnight
Yes. However, for written discovery, the California Code of Civil Procedure requires that the plaintiff wait 10 days after service of the summons or appearance by that party, whichever occurs first. Defendants do not have such waiting period and can serve discovery anytime (even before filing an answer).
I disagree with my colleague. If the defendant hasn't appeared, you need to wait the applicable waiting period, 10 days after service of complaint (CCP § 2030.020) and since you can't be sure what address to mail the discovery requests to and the defendant hasn't indicated one by an appearance, I would serve any discovery requests on them personally.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.