Does "proof of delivery" mean a list of those consenting e-banking customers to whom a notice of statement availability was sent? If so, then Regs E and DD both require "evidence of compliance" with the rules regarding periodic disclosures. In both cases, your "evidence" must be maintained for specified periods of time. In a paper-based environment, you are also required to provide "evidence of compliance" that statements went into the U.S. mail. In both cases your evidence will be procedural and the retention periods will be the same.
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...gone fishing.