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You can't get a divorce until the court is sure that your spouse is aware of the proceedings. Therefore, you must serve her with a copy of your petition. The rules for how you can do this vary somewhat from state to state. Many jurisdictions allow you to mail your spouse a copy of your divorce papers. If you are serving her by mail, using a post office box usually isn't prohibited if this is how she regularly gets her mail delivered, as long as you can prove that she actually received the divorce paperwork.
Service by Mail
You usually have two options for service by mail. If your divorce is amicable, you can send your paperwork to your spouse's post office box, along with an acknowledgment of service. When she signs and returns the acknowledgment, you can file this with the court as proof that she received your petition. If you send your papers by certified mail, return receipt requested, your spouse must sign the receipt, even if the mail is delivered to a post office box. You can then file a copy of the signed receipt, along with a proof of service form. If your spouse won't sign for the mail or won't sign an acknowledgment, you'll have to serve her another way.
Other Options
Serving your divorce papers by mail may be the easiest option, but it's not the only one. You can hire a private process server in most states, or ask the county sheriff or constable to deliver the paperwork for you. The server will provide you with a document that you can then file with the court, proving that the paperwork was delivered.