Zip code 30084 includes much of Tucker, but not all of it. It also includes part of the Northlake area and significant areas within Gwinnett County. Northlake Mall itself is located in 30345.
Zip code 30087 covers much of Smoke Rise, but no part of the City of Stone Mountain in DeKalb County. Its post office is located in unincorporated Gwinnett County, which is also referred to as Stone Mountain. The SRCA has worked with the USPS to allow Smoke Rise residents to use Smoke Rise, Georgia as their mailing address.
Zip code 30040 is commonly referred to as Doraville, even though that zip stretches way down into the north side of Tucker. No one in that area truly believes they are a part of Doraville.
Zip codes are service areas established by the US Postal Service for their convenience only. It has everything to do with delivery routes and mail volume, and NOTHING to do with political boundaries, county lines, cities, or neighborhood concerns. There is no relationship whatsoever.
Zip codes should not be used as some sort of validation or argument for the inclusion or exclusion of any particular area within any of the new proposed cities.