Frankly, there's no requirement that you notify customers of the fact they have gone overdrawn. Such a notice is often considered a good anti-fraud measure, though, since it has the potential for alerting a customer that something is amiss.
The closest that I know of to a requirement is the Best Practice included in the February 2005 Guidance on Overdraft Protection Programs:
"Promptly notify consumers of
overdraft protection program usage each
time used. Promptly notify consumers
when overdraft protection has been
accessed, for example, by sending a
notice to consumers the day the
overdraft protection program has been
accessed. The notification should
identify the date of the transaction, the
type of transaction, the overdraft
amount, the fee associated with the
overdraft, the amount necessary to
return the account to a positive balance,
the amount of time consumers have to
return their accounts to a positive
balance, and the consequences of not
returning the account to a positive
balance within the given timeframe.
Notify consumers if the institution
terminates or suspends the consumer’s
access to the service, for example, if the
consumer is no longer in good standing."
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John S. Burnett
BankersOnline.com
Fighting for Compliance since 1976
Bankers' Threads User #8