Where are my forwarded emails?

Open Mail Preferences - click on Accounts - Mailbox Behaviors. Look at the settings for storing sent messages and deleting trash. Set those options to hold sent mail and trash for a period that meets your needs. If those are sent to delete sent messages and to delete trash when Quitting Mail, you will not be able to find the forwards and replies the next time you log in.

If those options are set to Never, you'll grow unmanageable bloat in in your mailboxes and on-line storage. Pick a time - one week or one month - that meets your needs.

Try that and let me know if it works. 😉

Sent Box Behaviors set to Never delete sent items since I started. There are roughly 12,000 emails in there ....

So - makes me wonder, what is the indexing capability of Mail and/or Spotlight? (I do not know the answer to that one.)

Spotlight does find email messages by title or content. Why not try looking for those replies using Spotlight to search on the message Subject? Just a thought.

Are you configured to keep messages locally for off-line viewing and saved on the server? Spotlight is only searching the local mailboxes. I do not know if Mail looks both in Server bases mailboxes and local. Something for you to test and learn.

Some of web based email servers have their own maintenance and deleted schedules. My Gmail tells me to set all the local settings to "never", etc. and then the Gmail servers delete and clean up on their own schedules ignoring the settings in Apple Mail.

I am sorry I don't have a better understanding of your specific situation, but if you can use other tools (Finder -> Find and Spotlight) to locate the message replies, that might put you on a path to figuring out why Apple Mail cannot locate the messages.

You can post a new thread: "What is the maximum number of local messages that Apple Mail can index properly?" To me, 12,000 seems like a lot; but then I wrote my first data bases on 1 MB external floppy drives so I may have a warped sense of limits for current systems. 😉