If you mean are they required to deliver it by hand or the USPS in order for your bank to process it? No.
Your bank is a third party recordkeeper; i.e. you are neutral, you do not have a dog in this fight. If your customer wants to raise a procedural issue with the IRS she certainly can.
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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.