Bad formatting of emails, maybe because of accents or special chars (?)

Hi.
In advance, thanks for this plugin and thanks for making it free.
I am experiencing a problem using it with Gravity Forms on a client’s website.
Sometimes, some emails are received with errors in the HTML format, so they look bad, with parts of the code visible. Sometimes, a <s> tag appears at the beginning of the emails, which makes all the text look crossed out. And all emails are sent with spaces [& nbsp;] between the characters of the email itself, which had not been entered in the forms. The language of the forms and emails is Spanish, which includes accents and special characters such as “?” or “?”. I don’t know if this could be the reason. PHP 7.0 and GF and WP updated to the latest versions. I have already deactivated the automatic format in the content of the emails in Gravity Forms, but the error still happens in the same way.
When sending a test email, it is sent correctly, without any error.
I could provide you with emails, screenshots, the website and other related information, but privately, for reasons of confidentiality with the client.