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You have to be more precise. What is happening?
Can you describe step by step what is the user doing and at what point something wrong is happening?
Do they register, but are not getting any email?
Did you set the email correctly?
If yes, are you able to send email to yourself?
Do you have a Debug turned on? If yes, what are the messages? When do they appear?
There could be hundreds different issues, but we need to know step by step what is happening, so we can figure out what is going on.