Problem selecting email to delete

I'm trying to clean out my Mail inbox. It has more than 5,000 messages. I've run into a weird problem when finding emails to delete. Here's one example: I have lots of messages that, in the inbox, display the sender as "The New York Times." The subject line includes "On Politics: xxx xxx xxx." I select one of these messages, then click to display the sender's address. The sender address reads: "nytdirect@nytimes.***." I copy that address and paste it into the Search box. Result: "Found 0 results."

Huh? I'm seeing at least one such message in my inbox—it's right there in front of me—and I'm quite sure there are more. Yet Search can't find any of them. Not even the one I'm looking at. Surely I'm I doing something wrong, but what is it?

[Edited by Moderator]