Amazon does not allow users to send AZW3 e-books to a Kindle device via WiFi. However, I noticed that it is possible to send a MOBI e-book to a Kindle e-reader and still make use of the font bolding feature.
The solution is to use Calibre to convert an e-book to MOBI in both old and new formats. In Calibre, under MOBI file type output, select ‘both’ as the MOBI file type. After conversion, it is possible to send the e-book to a Kindle device via email or Send to Kindle.