As far as I'm aware you will never receive anything legitimate from ACH about "rejected" or "cancelled" transactions.

Here is the NACHA Press Release about ACH spam.

Further to previous notices since February 2011, NACHA has been the victim of sustained and evolving phishing attacks in which consumers and businesses are receiving emails that appear to come from NACHA. The attacks are occurring with greater frequency and increased sophistication. Perpetrators are sending these fraudulent messages to email addresses globally.

These fraudulent emails typically make reference to an ACH transfer, payment, or transaction and contain a link or attachment that infects the computer with malicious code when clicked on by the email recipient. The source addresses and contents of these fraudulent emails vary -- with more recent examples purporting to come from actual NACHA employees and/or departments -- and often including a counterfeit NACHA logo and the citation of NACHA's physical mailing address and telephone number.

NACHA itself does not process nor touch the ACH transactions that flow to and from organizations and financial institutions. NACHA does not send communications to persons or organizations about individual ACH transactions that they originate or receive.

Caution your customers not to open attachments or follow Web links in unsolicited emails from unknown parties or from parties with whom they do not normally communicate, or that appear to be known but are suspicious or otherwise unusual. Direct them to forward suspected fraudulent emails appearing to come from NACHA to abuse@nacha.org to aid in our efforts with security experts and law enforcement officials to pursue the perpetrators.

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