I was testing a new install and the registration details bounced from my host because of the way the mail_send was formatted in auth.php. If I changed the line (#364) to the following it works OK:
The bounce from the email recipient looked like this:
188.8.131.52 does not like recipient.
Remote host said: 553 Sorry, that recipient does not exist.
Giving up on 184.108.40.206.
and further in the response header:
Subject: Your DokuWiki password
To: James Culbertson<firstname.lastname@example.org