When sending email to multiple users, reinitialize the message for each

There is a minor privacy leak, specifically the line
  $message->addPart($nwbody, 'text/html');

does not replace an existing text/html part from any prior run, instead
it only adds another part. Suppose you are emailing 100 users. The 100th
user ends up receiving a message with one text/plain part, and 100
text/html parts, only the last of which pertains to that user. The other
99 text/html parts are for the 99 previous messages, and may include
sensitive information intended for those other users.