smtp not sending form content in admin email notifications

I use Gravity Forms and Send in Blue to manage several forms on my websites.

After the WordPress 4.5 upgrade, I started getting strange admin email notifications from my forms. The content that was put into the form does not show up in the admin notification email as it did previously. And whatever text does show up is repeated twice in the email message.

I checked with Send in Blue and they said I needed to verify the content at my smtp email client end, and that WordPress settings need to be checked.

They said I needed to eliminate the content type shown below from the email header to rectify the problem:

=====================================
Content-type (1): text/html; charset=UTF-8
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Can anyone tell me how to do that?

Thanks in advance!