Malware hit on /wp-mail-bank/lib/zend/mime.php

Hello!

While doing a malware scan on my site with Ninjafirewall, its scanner returned this malware hit with Mail Bank:

1-{REX}PHP.hexencoded.longstring.1: /home/user/public_html/my_site/wp-content/plugins/wp-mail-bank/lib/zend/mime.php

No other malware is detected except for the above. I installed Mail Bank via the WordPress back-end (Plugins -> Add New).

Is this a false positive or should I be concerned?

Thanks