In the configuration of PUR we has the possibility to configure a "registrationStrategy".
The Mail strategy info.magnolia.module.publicuserregistration.strategy.Mail is not working for several reasons:

• in this class the attributes (pagePath, emailTemplate, ect..) are never set, that trigger a NPE in Mail#validateRegistration() on the call of MailUtil.mail()

• in DefaultUserRegistrar.registerUser() the call to the strategy regStrategy.validateRegistration(user) passes a user where user.properties is empty. The values of this map are needed later in the MailUtil.mail().

Well it seems that this strategy is really not working.
Maybe we could drop it.
If we check the demo instance running on 4.4, http://demoauthor.magnolia-cms.com/demo-project/members-area/registration.html, the concept of the Mail strategy is substitute by the confirmation email message.