For comparison only, the NACHA rules for ARC entries (lockbox checks converted to EFT's) require the Originator to retain "a reproducible, legible, image, microfilim, or copy of the front and back of the receiver's source document for two years..."
However, they also require that the original document be destroyed within 14 days. As explained: This requirement is to protect against the risk that, by error, the source document might subsequently be entered into the check processing system for payment as a check.
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In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.