To get rid of the []
I modified the file Mailer.class.php as follows:
diff Mailer.class.php_orig Mailer.class.php
558c558
\< $prefix = '['.$conf['title'].']';
###
\> $prefix = $conf['title'];
560c560
\< $prefix = '['.\dokuwiki\Utf8\PhpString::substr($conf['title'], 0, 20).'...]';
###
\> $prefix = \dokuwiki\Utf8\PhpString::substr($conf['title'], 0, 20).'...';
563c563
\< $prefix = '['.$conf['mailprefix'].']';
###
\> $prefix = $conf['mailprefix'];