"subscribe by email" sends html codes to subscribers

Hello.
I'm using mailgun to send emails to the subscribers. It works well with wordpress usual emails and also with eNewsletter. However when I try to send emails to the subscribe using "subscribe by email", it sends html codes like a plain text.
Please take a look at the attachment.
I searched in the interent and found these threads:
https://premium.wpmudev.org/forums/topic/subscribe-by-email-plugin-raw-html?utm_expid=3606929-87.FQUx5sKvRhKbhK_8_C59WQ.0&utm_referrer=https%3A%2F%2Fwww.google.com%2F