Nope. Aggregating unclass data to make classified is (comparatively) rare. In about 8 years I saw it..... maybe three times. Another guy here wrote a post about it happening in his career... once.
What is likely happening with the 305 emails is a series of dirty word searches and SME searches.
EXAMPLE ONLY
So, for example, if an email without a classification stamp says "Hey, isn't General Smith going to launch a coup against Prime Minister Jones if he tries to impose budget cuts on the military?", then that information needs to go to the DIA, the CIA, the NSA, et. al. to see if it came from a defense source, a cia source, a sigint source, or what, and what the original classification was.
Another layer of complication can come when you have imperfect match between a document whose classification you are trying to ascertain, classified reporting, and unclassified reporting. So, for example, if we
did have a highly placed HUMINT source that had confirmed General Smith's evil plans, which was SECRET//HCS//NOFORN, but also an article from Reuters which includes
"Coup rumors have been circling the capital of Arcadia as Prime Minister Jones prepares for a showdown with the military... General Smith was quoted as saying that Prime Minister Jones should put first emphasis on defending the country, or the results could be disastrous"
then you also need to check up on the security clearance of the individual who generated the unclassified email. If they had access to the SECRET//HCS//NOFORN report, then they've just put something classified that they read onto an unclassified email. If they have no clearance, or are not cleared for HCS, then it is possible they didn't get that information from classified sources, and are simply taking a wild assumptive leap off of the Reuters article.
/EXAMPLE ONLY
I've dealt with a few FOIA requests where you have to go back and search through all your own records for "Anything Pertaining To X", etc. It's a pain. My (uninformed) bet is that the process looks similar, but with a few extra database searches.
Two emails of unclass in an inbox cannot aggregate together to make classified unless you paste the one into the other, or put information from both into a third document.
I sure as hell do. :lol: It's probably completely unnecessary given the context and the discussion, but I'd rather err on the side of caution than ever get caught up in a dirty word search and hauled in to explain myself.