Looking at this Stack Overflow answer I would suggest adding the 'Content-Type: text/plain; charset=UTF-8' header to the 'body' parameter in the call to the mail function.
It seems from this post that support for unicode email addresses in the local part is relatively new. e.g. Postfix supported it in 2014. It very much depends on what email software your server (or ISP) has installed to support the sending of email. I suspect that if you use gmail, or google apps for business then you wouldn't have any problems (assuming the content header is set correctly). Using Postfix for example you would need to ensure that version 3.0 or greater is installed. In the debian world this would be Debian Stretch. Wheezy and Jessie are both on Postfix 2.x. You can find further information on SMTPUTF8 support and links to relevant RFC on the Postfix SMTPUFT8 page.
To be safe, I would suggest that all email addresses in *your* organization use the 128 lowercase ascii characters as required under the old standard.