Notification not works after submission

I noticed, in a specific case, that after submitting the form, the notification is not sent.

Reducing the length of the text in the message field (paragraph text field) solves the problem. Is there a way to understand what generates the problem? Is it a problem of text length? Can there be something in the text that creates this problem?

There are no active spam filters, and in any case the entry is regularly recorded and processed (add-ons works fine).