> Why use a default mailer@xwiki.localdomain.com which has no chance of working?

I do not understand, it works when the admin email is empty, I tested it.

> Why not instead check if it's defined and it not report and error and ask the user to define the admin email address?

For not penalize the usability of the application if the admin is not here to add the mail.  But I can compose an email with the domain name instead of this static email.