As standard, when you create a new user in Joomla! that user gets an e-mail to tell them their user details.
This is super useful… Unless you don't want them to get an e-mail..!
Then you've gotta try to find how to stop it…
Tricky!
But I found it…
Go to Plug-in Manager on the Extensions menu.
Then use the search feature to find the User - Joomla! plugin.
In the Basic Options of this plugin you can enable and disable "Notification Mail to User".