Requesting a statement of purpose for a wire transfer is due diligence at a fundamental level. At one end of the spectrum, community banks, it is rarely required. At the other, some banks not only require a statement of purpose, but an invoice in support of that statement of purpose.
Give your customers advance notice of the change in procedure along with an indication that there will be no exceptions. If you want to wiggle your toes in the water a little first, start by requiring it on international wire transfers first.
If your wires can be organized in a spreadsheet format you can compare the purposes your customer has claimed over a period of time...
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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.