According to the latest code for the email handler:

        // Add plain text alternate view.
        var mimeType = new ContentType(MediaTypeNames.Text.Plain);
        var emailView = AlternateView.CreateAlternateViewFromString(plainTextBody, mimeType);
        message.AlternateViews.Add(emailView);


        // Add HTML alternate view.
        mimeType = new ContentType(MediaTypeNames.Text.Html);
        emailView = AlternateView.CreateAlternateViewFromString(htmlBody, mimeType);
        message.AlternateViews.Add(emailView);

It should be including both an HTML and a plain text version of the email. See here: https://github.com/rhythmagency/formulate/blob/v3/master/src/formulate.app/Forms/Handlers/Email/EmailHandler.cs

If by "plain text" you mean that the emails are very basic, you could either create a custom handler to send a more developed email or you could use the designed emails feature of Formulate pro: https://www.formulate.rocks/pro/designed-emails

That allows you to construct custom emails (e.g., if you want to have a better design).

Another alternative would be to send the data to some third party like HubSpot, then send an email using that platform.

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