This is actually a combination of the certified mail requirement of 6212, and evidence rules re: certified mail. These evidence rules are discussed in some cases re: NOD's where a dispute arose as to mailing/receipt.

However, in many CDP cases the hearing officer is so sloppy they sometimes don't even discuss NOD's at all, much less proof of receipt of them. Since an NOD is a pre-collection procedural requirement, and 6330(c) requires confirmation of compliance with all procedural requirements, I think that puts the burden of proof on them to show evidence of receipt of an NOD.