FIX Introduce hidden option MAIL_PREFIX_FOR_EMAIL_ID to solve pb of

tracking email.

htdocs/core/class/smtps.class.php

@@ -1126,6 +1126,8 @@ class SMTPs
 		$host=preg_replace('@tcp://@i','',$host);	// Remove prefix
 		$host=preg_replace('@ssl://@i','',$host);	// Remove prefix
 
+		$host.=dol_getprefix().'-'.$host;
+		
 		//NOTE: Message-ID should probably contain the username of the user who sent the msg
 		$_header .= 'Subject: '    . $this->getSubject()     . "\r\n";
 		$_header .= 'Date: '       . date("r")               . "\r\n";