Exception holds, including large deposit hold, can be applied against US Postal Money Orders.
The USPS has also advertised the following, with a number to call at the end for verification:
oney Order Security Features
Things to check for to ensure a Money Order is authentic.
Postal Money Orders are safe to use because they have many security features that help distinguish between real and counterfeit notes. Just like our own U.S. currency, they are specially designed with different colored inks, watermarks, and security thread in the paper.
How to tell if it’s official
Hold the money order up to the light and look for the following security features:
Ben Franklin images (watermarks) repeated on the left side (top to bottom).
A vertical, multicolored thread that weaves in and out of the paper to the right of the Franklin watermark. When held to the light, the thread appears continuous with alternating horizontal dark and light bars behind it, and the letters USPS repeating backward and forward throughout the thread.
If either of these security features is not present when the money order is held up to the light, you are holding a fraudulent Postal Money Order.
Also be aware that
Denominations are indicated on two locations.
Discoloration of the denomination amounts indicates erasure (fraudulent).
Maximum value for domestic postal money orders is $1,000.
Maximum value for international postal money orders is $700. (Maximum for Canadian money orders is $1,000)
Notice 299 take-ones (highlighting money order security features) are available to order from the Topeka Materials and Distribution Center (1-800-273-1509). Or, you can download the Notice 299 (pdf) (htm) (October 2008).
To verify a postal money order, call the Money Order Verification System at 1-866-459-7822. If you suspect fraud, call the U.S. Postal Inspection Service at 1-877-876-2455 (select option 4).
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I hear and I forget. I see and I remember. I do and I understand.--Confucius