Run: postconf "smtputf8_enable = no" postfix reload And the next time, enter your question into a search engine of your choice and follow the instructions you find there. I'm not your search bot just because you are too lazy.
Ok, then you go to the postfix page about smtputf8 and what do you find there? http://www.postfix.org/SMTPUTF8_README.html I'll cite:
So you want to update postfix to version 3 to disable smtputf8? Makes not really sense as smtputf8 is not implemented in your current version which means it is disabled, so nothing would change in regard to disabling smtputf8. Maybe you should consider updating to a more recent Ubuntu version if you want to more recent software versions, Ubuntu 14.04 is quite old.