Unable to filter forwarded gmails

Here’s an odd little thing. I’ve accumulated numerous email addresses over the years, and currently they all forward to my gmail account, from which I can send using any one of them. But it’s actually impossible to reliably filter them on the original address. If the email was originally sent via blind-carbon-copy, then forwarded to gmail, only the ‘delivered-to’ header contains the correct information. But it isn’t searchable.

A small thing, really, but it’s strange that such information isn’t accessible. Not much of a problem for me, but today I was working on an account with hundreds of forwarded emails, and could find no way to separate them. I ended up turning off forwarding and using the Mail Fetcher, which can automatically assign labels to individual accounts.