Related to http://tracker.phpbb.com/browse/PHPBB3-10184.

Using a url like this:

http://localqi/boards/y1/ucp.php?i=pm&mode=compose&u=1

Anonymous is selected as a recipient.

However, attempting to send an email produces:

No recipient defined.

Thus it is not possible to send pm to anonymous, but it is possible to select anonymous as a recipient via url.