There are two different workarounds I can suggest, info485.
The default sender address of QuickBooks Online is quickbooks@notification.intuit.com. You can let your customer to create a rule. This will let all the upcoming emails from that address be delivered to a specific folder or to the inbox. They can reach out to their email provider for an assistance on how to create one.
Another option is to add the quickbooks@notification.intuit.com to their contact list or address book. This is to tag it as "safe sender" and won't be mark as spam. Check this article about receiving important email messages from Intuit. They can follow the same steps on the Add approved email adresses for your email provider section.
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