45 CFR 1303.5 - Service of process.
Whenever documents are required to be filed or served under this part, or notice provided under this part, certified mail shall be used with a return receipt requested. Alternatively, any other system may be used that provides proof of the date of receipt of the documents by the addressee. If this regulation is not complied with, and if a party alleges that it failed to receive documents allegedly sent to it, there will be a rebuttable presumption that the documents or notices were not sent as required by this part, or as alleged by the party that failed to use the required mode of service. The presumption may be rebutted only by a showing supported by a preponderance of evidence that the material was in fact submitted in a timely manner.