Is there any solution (plugin), which would allow me to accept form submissions from specific domain and create new topics for them? It would be even better, if I could reply people by responding inside the Discourse.
I have done this in the past. There was another application receiving a form that had product reviews, and after receiving the form would create a new topic in Discourse with the review under a specific category.
The Discourse and sister site had SSO, so we could share logins and create topics under the reviewer Discourse account.
This is all doable under the API.
You might check out the custom wizard plugin.