My first thought is that perhaps the journal owner/community maintainer has voluntarily turned on the require-captcha requirement, or that it has incidentally turned on due to high comment volume.
My thought is further that the bounceback is the symptom of the captcha requirement, and that it may do this any time it hits that.
no subject
My thought is further that the bounceback is the symptom of the captcha requirement, and that it may do this any time it hits that.