Even though you are not sending emails yourself (it's done by the server script from that contact form), you still might want to use the following PHP-script on your website to collect domains of sites where you have sent a message but who are not interested. This would prevent any further problems and not send there again. Instructions are also in the script for further details.
<?php
/*
1. Upload this file to your webserver and call it e.g. unsubscribe.php
2. In GSA Website Contact -> Edit Project ->Submission Content->Message add something like...
Click the following link to unsubscribe: http://www.mywebsite.com/unsubscribe.php?d=%domain%
3. in GSA Website Contact -> Options -> Global Filter -> add the URL to your $blacklist_filename from below
http://www.mywebsite.com/blacklist.txt
*/
$url_parameter_for_domain="d";
$blacklist_filename="blacklist.txt"; //the filename where it is stored and where the program can access it to read unsubscribed data
//a message given to the user
$message_title="Successfully unsubscribed";
$message_body="We apologize for any inconvenience. We will no longer contact you in the future.";
$message_redirect_url=""; //if set, it will go to this URL after a short time
//------------------ the code starts below -------------
//check if a special GET parameter is present, if not, set it via javascript and reload
//this prevents that bots will crawl this URL and add domains to the blacklist
if((!isset($_GET['real']) || ($_GET['real']!="yes")) {
echo "<script>let url=window.location.href;if(url.indexOf('?')>-1){url+='&real=yes'} else {url+='?real=yes'};window.location.href=url;</script";
} else {
//get domain and validate
$domain=$_GET[$url_parameter_for_domain];
if(($domain!="") && (preg_match('/^(?:[-A-Za-z0-9]+\.)+[A-Za-z]{2,6}$/',$domain))) {
file_put_contents($blacklist_filename, $domain."\n",FILE_APPEND);
}
}
//output some message
echo "<html>\n".
" <head>\n".
" <title>".$message_title."</title>\n".
' <meta charset="utf-8">'."\n".
' <meta name="viewport" content="width=device-width,initial-scale=1">'."\n".
" <style>\n".
" *{margin:0; padding:0; box-sizing:border-box;--textcolor:#fff; --bgcolor:#121212; --highlight:#2749c9;}\n".
" @media (prefers-color-scheme:dark){*{--textcolor:#141414; --bgcolor:#f0f0f0; --highlight:#ff7f00;}}\n".
" body{font-size:18px; font-family:system-ui,sans-serif; line-height:1.4; color:var(--textcolor); background:var(--bgcolor); position:relative; max-width:64em; margin:0 auto;}\n".
" header{padding:5vw 5vw 0 5vw; display:flex; flex-wrap:wrap; position:absolute; width:100%; z-index:2;}\n".
" header h1{font-size:1em; flex:1;white-space:nowrap; padding:0 5vw .5em 0;}\n".
" p,ul,ol,article{max-width:60ch;margin-bottom:.6em;}\n".
" </style>\n";
if($message_redirect_url!="") echo ' <meta http-equiv="refresh" content="5;url='.$message_redirect_url.'" />'."\n";
echo " </head>\n".
" <body>\n".
" <br><br><h1>".$message_title."</h1>\n".
" <br><p>".$message_body."</p>\n".
" </body>\n".
"</html>";
?>