This bug is present in Release 2012-10-13 "Adora Belle"
The following log snippet is from Windows Server 2008 R2 SMTP server, you can see that after the valid email address a second RCPT is added which is empty. This causes the 501 error response and subsequently the email is not sent.
2013-01-17 19:18:16 127.0.0.1 SMTPSVC1 127.0.0.1 HELO - +DEV01 250 0 51 24 0 SMTP - -
2013-01-17 19:18:16 127.0.0.1 SMTPSVC1 127.0.0.1 HELO - +DEV01 10.10.10.70 0 MAIL - +FROM:<
noreply@dokuwiki.intranet> 250 0 62 49 0 SMTP - -
2013-01-17 19:18:16 127.0.0.1 SMTPSVC1 127.0.0.1 HELO - +DEV01 10.10.10.70 0 RCPT - +TO:<
andy@dokuwiki.intranet> 250 0 30 27 0 SMTP - -
2013-01-17 19:18:16 127.0.0.1 SMTPSVC1 127.0.0.1 HELO - +DEV01 10.10.10.70 0 RCPT - +TO:<> 501 0 27 10 0 SMTP - -
The proposed code change above does fix the problem.