As for the "I am sure you didn't mean to" communication, use caution. As Randy suggests, your bank should not accuse the customer of breaking the law. If you want to do something, send the customer a letter with a copy of the FinCEN pamphlet on CTR filing, which includes some limited information about the law against structuring. Your cover letter can be fairly generic, saying something like "we want our customers to be familiar with certain federal law requirements that may affect them."
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John S. Burnett
BankersOnline.com
Fighting for Compliance since 1976
Bankers' Threads User #8