The taxpayer is advised their refund check was mailed undelivered by the Postal Service. The taxpayer is provided the option to update their address online. Check account to determine if the taxpayer changed their address online. If an address is changed, advise taxpayer their request is being processed. If a taxpayer did not change their address, follow instructions in IRM 21.4.3.4.3, Undeliverable Refund Checks.

If you check Where’s My Refund (WMR) tool and see you have a reference code, you should identify the code and see if any action is required.
In many cases, no action is required on your part. If there is an issue, the IRS will typically send you a letter to your mailing address on file within 90 days stating what the issue was, and any additional information required.