Can Certified Mail be sent to Military Post Offices and Diplomatic Post Offices?

Yes. Military Post Office addresses (APO and FPO) and Diplomatic Post Offices (DPO) are considered domestic mail.  However, the Certified Mail service is not available at all APO, FPO, And DPO addresses.  

How do I know if the Certified Mail service is available at my MPO/DPO address? 

Certified Mail Labels.com will not create a label if the service is not available at the delivery point address.  You'll receive an error message saying one or more services is not avaialbe if the Certified Mail service is not available at the delivery point address. 

Why can't I use an APO, FPO, or DPO address in my Mailing Profile as my Return Address?

When a label is created, USPS verifies if the Certified Mail service is available at the delivery point address. It does not verifiy if the Certified Mail service is available at the Return Address.  Therefore, APO, FPO, DPO addresses cannot be used for the return address due to the possilbe instance of a Certified letter being rejected by the recipient and unable to be returned to sender.  All Certified Mail must be signed for at delivery whether that delivery occurs at the delivery address or the return address.  If a Certified letter cannot be signed for in return to sender instance, it would be considred a dead letter and would be lost.