ATL is a BIG airport, with multiple concourses, and DL is their largest airline. If you pre-clear US Immigration and Customs in YYZ then your flight could arrive at one of several different concourses. Likewise, your connecting flight could depart from one of several different concourses, but probably E or F. ATL has a good underground train system which connects the concourses. If your connecting flight is in a different concourse then just make sure that you take one going in the correct direction.