I had the same problem. For me, the problem was caused by a different from usual setting of my blog charset. I used UTF8_unicode_ci in my database. You could check yours under wp_options in your database.
And the direct cause of the problem in question is the misbehavior of the function html_entity_decode, which appeared twice, around Line 821 and 836 in the document “mailpress\mp-includes\class\MP_Mail.class.php”.
A little reference to the function html_entity_decode could be viewed here: http://php.net/manual/en/function.html-entity-decode.php
I guess, in those two lines, instead of retrieving the charset from your database, configuring directly (e.g. ‘UTF-8’) could solve your problem. At least, it solved mine.
Good luck.
