The xapian-discuss and xapian-devel mailing lists are the primary point of contact for the project. Please don't mail individual developers directly unless explicitly requested to. Mailing just one developer means you've less chance of getting an answer than mailing all the developers!
Similarly, please reply to the mailing list rather than privately. If you're getting free help and advice the least you can do is make sure the whole message thread is in the list archive to help others in the future!
If you want private one-to-one assistance and are prepared to pay for it, consultancy services are available.