Are referendum results binding?

Estonia

Answer

Mandatory referendum - always
Optional referendum - always

Source

Constitution

http://www.president.ee/en/republic-of-estonia/the-constitution/index.html

? 105. […]
A law which is passed by a referendum shall promptly be proclaimed by thePresident of the Republic. The decision of the referendum shall be binding onall state institutions. If a bill which is submitted to a referendum does notreceive a majority of votes in favour, the President of the Republic shalldeclare extraordinary elections to the Riigikogu.

ReferendumAct

https://www.riigiteataja.ee/akt/1045564