Mail header is not supported multi-byte character.

* actions/invite.php
* lib/channel.php
* lib/mail.php
* lib/mailhandler.php
* plugins/Imap/imapmailhandler.php

In the above files, it is necessary to apply mb_encode_mimeheader(mb_convert_encoding(XXXXX, 'utf-8')) to the right side value of $headers['From'], $headers['To'] and $headers['Subject'].

In addition, we should add the following code to mail.php :

if (!isset($headers['MIME-Version'])) {
$headers['MIME-Version'] = '1.0';
}