Fred, like you I would be very suspicious if it's legit. I received an email recently, although not in regard to the US Postal service, from a company claiming my domain names are in jeopardy if I don't contact them right away. I own several domain names associated with my previous fly tying business & keep them registered & parked with Go Daddy. My first thought was that they were phishing scams as it directed me to fax information to a phone number.
I forwarded the email to Go Daddy & the reply indicated it was a legit company trying to get me to re-register my domains with them, at a significantly higher cost.
This companies scare tactics don't set well with me! But, I have to wonder how many other folks will take the email as a true concern & accept the "offer"!
There is some legal-eeze in their email, that states you're not obligated "UNLESS" you accept the offer & fax the info they request! It's a play on words attempting to fool those they send it to. It's still a scam IMO, although it appears to be legal.
Be careful, & IMO, you're doing the correct thing by going to the USPS directly!