"Use the source, Luke"

From Net::SMTP's source code:

 my @ok;
 my $addr;
 foreach $addr (@_) 
  {
    if($smtp->_RCPT("TO:" . _addr($smtp, $addr) . $opts)) {
      push(@ok,$addr) if $skip_bad;
    }
    elsif(!$skip_bad) {
      return 0;
    }
  }

 return $skip_bad ? @ok : 1;
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Which tells us that

@goodrecips=$smtp->recipient(@recipients, 
                 { Notify => ['FAILURE'], SkipBad => 1 });
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Will return the list of valid recipients and return notifications of failures (DSNs) via SMTP. Of course, some or all of this may not work, depending upon the configuration of the remote SMTP server.