D.C. Mun. Regs. tit. 25-D, § 1503
1503.1 Proof of proper service may be made by certificate of service signed by the person making service or by admission of a return receipt, certificate of mailing, or a written acknowledgment signed by the licensee, the person operating a massage establishment or health spa facility without a license, or an authorized agent.
Source: Final Rulemaking published at 56 DCR 7975 (October 9, 2009).