Duncan Smith also appears on both the 1820 and 1830 Federal Census of Wayne County, Mississippi.
In 1820 one male child is listed, and in 1830 five children are listed, four males and one female.
Jasper County was created in 1833 from Jones and Wayne County, but mostly from Jones County.

A Few Postmasters of Mississippi City in Harrison County:

Noah Ligon.............................July 1, 1852 to December 31, 1852
Robert Cowan.........................January 1, 1853 to June 30, 1853
I am not sure who served between July 1, 1853 and June 30, 1854.
Salatheil Smith.......................July 1, 1854 to September 30, 1854
William M. Burton or Barton.....October 1, 1854 to September 30, 1860
Joseph Barrow........................October 1, 1860 to June 30, 1861

Source:
Register of Officers and Agents
Civil, Military, and Naval
in the Service of the United States
for 1853, 1855, 1857, 1859, 1861

Best regards----