The work that happens before anyone presses send.
Who to leave out is settled at the same moment as who to include. Records are aged as well as counted, because staleness varies by field. You get the rules as well as the rows, so the build can be repeated.
Change log
A note of what each processing stage altered, kept so the build can be retraced.
Field standardisation
One definition per column, applied consistently, with original values retained beside the normalised ones.
Phone formatting
Rendering numbers in a consistent international form, with the original retained.
Start with one segment rather than the whole plan, and see whether it holds.