Howdy,

How can we configure a welcome email for new registered users?

We have the Users need to validate their account option disabled because it doesn't help us. We lose a lot of users because they don't see the email.

Now the account is automatically activated, but we would also like to receive a welcome message by mail.

Thank you!

I am oretty sure there is mail of type "Thanks for your registration".
If you did not get it, review mail server settings

The mail settings are ok because when we activate "Users need to validate their account" all the users receive the emails after registration with the confirmation link, but we want to keep deactivate this option, and the user receive an welcome email. At Email Templates we have email_user_registration separated by email_user_validation but we cannot find out how to activate email_user_registration.

Check this one

Yes, we configured correct, when we test it it comes on the test email but if any user make an account did not receive this email.

Try to enable error logging and check if there are not some error generated during registration.
But really I do not remember if this email is sent always, but I think yes.