If you save an attachment from an e-mail message, and later need to find exactly which message it came from but cannot remember which it was, it can be done with a little bit of work.
First, select the file in the Finder and open its Info window by pressing Command-I. In the More Info section, you should see the file's "Where from" information. This will list the sender's name and address and the subject line of the message that the file was originally attached to. You can first use this information to try searching in Mail for the message. Many times, that's all you'll need to do.
However, if that doesn't cut it for you - for example, you have multiple revisions of a file that were sent back and forth in a series of messages, and you need to determine which came from which - there is one more bit of metadata in the "Where from" information that can solve that for you.
Copy the part of the "Where from" information that looks something like this:
message:%3CCD253A37.36E7E%25sender@example.com%3E Switch to Safari and paste that into the address field. Mail will activate and show you the message (assuming it still exists).
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