possibility to check one domain ?

Hello @yberges
the processor checks if the domain entered is most likely capable to recieve eMails. There is no checking for a particular domain or if the mailbox exists. But you can take the code and adopt it to your needs. The quick and dirty (=hardcoded) version would be to add the relevant code in file: cf-email-check.php in function EDCCF_email_domain_has_no_mx( $value ). You can of course extend the coding for an additional user input field to be more flexible.
Hope this helps.