Welcome message in this plugin is only intended to be sent as an instant message to a new user when he logins for the first time. Also many clients don’t even have an email field in the registration dialog (e.g. Spark, Pidgin, Psi don’t). Though some has (e.g. Gajim).
I have the registration webpage enabled, I thought the settings in the plugin for a welcome message meant that an email would be sent to the new user when they registered on the webpage. Apparently this is incorrect although this would be a nice feature to have. Since I was testing I never actually logged in as the user I was creating and never got the message I set up which I had assumed was being sent in an email