diff -ru ./typo3.orig/trunk/t3lib/class.t3lib_htmlmail.php ./typo3.new/trunk/t3lib/class.t3lib_htmlmail.php
--- ./typo3.orig/trunk/t3lib/class.t3lib_htmlmail.php	2010-03-01 12:09:02.986059652 +0100
+++ ./typo3.new/trunk/t3lib/class.t3lib_htmlmail.php	2010-03-01 13:58:51.286024540 +0100
@@ -723,17 +723,27 @@
 			return false;
 		}
 
-			// On windows the -f flag is not used (specific for Sendmail and Postfix),
+			// On windows the -f flag is not used (specific for Sendmail),
 			// but instead the php.ini parameter sendmail_from is used.
-		$returnPath = (strlen($this->returnPath) > 0) ? '-f ' . escapeshellarg($this->returnPath) : '';
-		if($this->returnPath) {
-			@ini_set('sendmail_from', t3lib_div::normalizeMailAddress($this->returnPath));
+		$returnPath = ($this->forceReturnPath && strlen($this->returnPath) > 0) ? '-f ' . escapeshellarg($this->returnPath) : '';
+		$returnPathMayPossible = false;
+		if (TYPO3_OS == 'WIN') {
+			$returnPathMayPossible = true;
+			if ($this->returnPath) {
+				// This works only on Windows. On Unix this _might_ lead to 500 errors
+				@ini_set('sendmail_from', t3lib_div::normalizeMailAddress($this->returnPath));
+			}
 		}
 		$recipient = t3lib_div::normalizeMailAddress($this->recipient);
 		$recipient_copy = t3lib_div::normalizeMailAddress($this->recipient_copy);
 
 		// If safe mode is on, the fifth parameter to mail is not allowed, so the fix wont work on unix with safe_mode=On
-		$returnPathPossible = (!ini_get('safe_mode') && $this->forceReturnPath);
+		// On unix, only a scrupulous well configured sendmail MTA accepts the -f parameter. The currently popular postfix MTA
+		// refuses this as well as other MTA like exim or qmail due to popular separaration of user privileges.
+		// Indeed, -f _can_ be configured for the cost of potential overall system vulnurability.
+		// To get a crossplatform functional framework, the PHP mail() function should be marked deprecated and a class should
+		// be introduced to perform SMTP like pear net_SMTP
+		$returnPathPossible = (!ini_get('safe_mode') && $this->forceReturnPath && $returnPathMayPossible);
 		if ($returnPathPossible) {
 			$mailWasSent = mail($recipient,
 				  $this->subject,