When using the mass email function in the ACP to send a message to more than two members the operation appears to fail when address hiding and the use of PHP's mail() function is enabled. However, the emails successfully arrive to the intended addresses after about an hour. Both servers I've observed this on use Sendmail as the MTA (versions 8.11.6 and 8.14.2). The problem appears to be with the use of "undisclosed-recipients:;" in the "To:" field when nothing has been specified for the $to variable (includes/functions_messenger.php line 400). Sendmail interprets "undisclosed-recipients:;" as a literal address and of course bounces its attempt to send the message to it, saying "553 5.1.3 undisclosed-recipients:;... List:; syntax illegal for recipient addresses". As a result PHP reports that the call to mail() failed even though the copies to be delivered to the actual members are successfully queued by the MTA