Hello. I sent a notification about your question to the company employees, expect an answer from them here in the comments.

Perhaps you should try to create different mailing addresses for sending letters from the site and use the SMTP in the program settings:

Dear Aleksej 

I follow your instructions.

Unfortunately same, a confirmation email with mailed-by: website A, not website C and to the spam folder

Rgds,

Nek

Dear Aleksej,

Some providers his completely out of reality.
He tells me to use the -f parameter and to ask IT, programmer, to do it in my PHP file, with results every time I upload the website must add these settings. 
Really how some people do this job? Just he checks in google and sends me a link to know what I have to do.
Is my first time with Plesk control and I do something very easy...
Login to Plesk control
go to PHP settings and so easy in mail.force_extra_parameters I add ***
Now page email going to inbox, confirmation email going to inbox with mailed-by: the correct domain

Thanks for your advice 

Rgds,

Nek