In the image you can see all the plugins used on the site.
Thank you for your response.
I have attached a screenshot of the Mail tab settings (email addresses have been replaced with fake addresses for privacy).
Additional information:
- if no attachment is uploaded, the message body is delivered correctly,
- as soon as the attached file is included, the email body becomes empty,
- the attachment itself is still delivered successfully.
The problem started right after the latest CF7 update.