Huh, maybe your use case is around the indexing of the contents of attachments? I basically never search for the contents of attachments, just the clip does of emails, and have found gmail search to be really good. I switched back to the web client from Mac’s native mail app for this reason because search has been so good for me in Gmail.

I haven’t looked, but I wonder if there is a good hackable email client that will let you substitute out the search index with a reasonable abstraction from all the complicated email protocol stuff. I feel like building an index for your use case is totally achievable if so.