Corpus Christi Fire Department officials say nobody was hurt in the accidental blaze and the building likely will have to be demolished.
Capt. James Brown says the fire appears to have been electrical in nature and started near an appliance.
U.S. Postal Service spokesman Sam Bolen says about half of the 800 individual mail boxes were damaged in Tuesday night's blaze. The fire in the Annaville neighborhood affected mail in the 78460 Zip code. Workers have been sorting through damaged mail and rerouting items to an alternate post office.
The Postal Service doesn't cover losses for destroyed mail unless the parcel was insured.
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