Message application or SMS sending in Jolla's SailfishOS core does not encode full Unicode character set properly. Thus sending Emoji mostly fails. Receival from other phones works perfectly except few symbol sent from iPhone have variant selector in use and Jolla can't process that properly and thus shows correct Emoji symbol but also square box after it.

Mitäkuuluu is processing those variant selectors correctly.

The Emoji Keyboard only includes means to write standard Unicode Emoji and a customized font to present all Emoji symbols correctly. It's then up to application and data transport if UTF-8 encoding is fully implemented.