The PutEmail processor makes use of the Jakarta Mail library. The does not allow specifying the character set used for the email subject, though it does for the message body. As a result, if there are unicode characters, they do not come across properly in the subject but instead have a bunch of question marks instead.
However, the issue appears to be resolved by updating conf/bootstrap.conf with the following:
Given that we make UTF-8 the default everywhere else that we have the ability to specify it, we should include this by default in bootstrap.conf