Submitting to WordPress.com "Email Post"

My form works perfectly, until I change the recipient address to the email created for wordpress.com email post submission tool. I guess I should set up some parameters properly, but have no idea which and how.

Broader picture: I am making a crowd-sourced image collection. What I need is a form for anonymous image submission, with several firlds, later to be included in the post.

Native wp.com contact form has no file attachment option, so I set up a form on separate free WP hosting (no possibility to set the whole website there) just to let people submit pics for screening. Everything works fine, until I change delivery address to point to WP.com

Please advise.

https://wordpress.org/plugins/si-contact-form/