Envelope sender disappeared during the refactoring of send between 5.3 and 5.4 and therefore was set using the from address. This led messages to be identified as junk by SPF, and bounces to be sent back to the user. This is a fix that puts the correct envelope sender back.
Since acs_mail_lite::send now accepts a multiple TO address, the user_id part of the bounce address it set to 0 if there are multiple recipients.