A RDBMS can provide a fast full text search alongside other useful features, can you elaborate how it's orthogonal?

Oh, and I also dislike even the idea that I have to find some "specialized software" to have basic functionality in my e-mail client. That's why most still suck a*.

You probably want something that checks for similarly spelled words, maybe natural language processing, whole or partial word matching, indices on all this, etc.

If all you want is to grep your email, that's different, I think you can accomplish that with pine or other text mode email clients.