diff --git a/htdocs/core/class/CMailFile.class.php b/htdocs/core/class/CMailFile.class.php
index d766fa4a8132f47bb431c44d9fb58dd04fc82501..ed9464a14b3f2a7839e92b53834e6cdf90cab1a4 100644
--- a/htdocs/core/class/CMailFile.class.php
+++ b/htdocs/core/class/CMailFile.class.php
@@ -657,6 +657,7 @@ class CMailFile
 		{
 			$out.= "To: ".$this->getValidAddress($this->addr_to,0,1).$this->eol2;
 		}
+		// Return-Path is important because it is used by SPF. Some MTA does not read Return-Path from header but from command line. See option MAIN_MAIL_ALLOW_SENDMAIL_F for that.
 		$out.= "Return-Path: ".$this->getValidAddress($this->addr_from,0,1).$this->eol2;
 		if (isset($this->reply_to)  && $this->reply_to)  $out.= "Reply-To: ".$this->getValidAddress($this->reply_to,2).$this->eol2;
 		if (isset($this->errors_to) && $this->errors_to) $out.= "Errors-To: ".$this->getValidAddress($this->errors_to,2).$this->eol2;