Earlier quoted context omitted.
their definition (via secret interpretation of USSID 18, according to Bill Binney[1]) is roughly that it's only collected if they actually go back and look at it at some point down the line. your web browsing history (or whatever) sitting on a hard drive in utah doesn't fall under this definition until they look at it. [1] https://youtu.be/r9-3K3rkPRE
So all it would take is for them to change their policy at any point in the future, even only for a few minutes, and instantly "collect" all current and historical emails from Americans.
If Congress doesn't like these retention / collection interpretation of their laws, they can clarify them in statute at any time. But they haven't and probably won't, because "retain everything for judicially approved retroactive collection" is incredibly useful e.g. when a bomb goes off at a marathon and a judge OKs the suspect's communications being reviewed.