The SMTPUTF8 RFC to support non-ASCII local parts is from February 2012 and thus fairly new [1]. Postfix supports it since July 2014 [2] and to the best of my knowledge Dovecot still does not yet support SMTPUTF8 for LMTP.

Personally I did not enable SMTPUTF8 support in my Postfix due to the lack of Dovecot support.

[1] https://tools.ietf.org/html/rfc6531 [2] http://www.postfix.org/SMTPUTF8_README.html