Email was not sent with Open Registration

I have test a DF2.0 instance in online sandbox environment.
I configure Open Registration with email confirmation. (Open Email Reg Service Id is 5, by default).
User can see the correct page of registration with email confirmation. But no email is sent to user’s mail box.
Does anyone can help me about this issue?

Can you provide your instance URL? You can PM it to me if you don’t want to post it publicly. I can look at the log files to determine if the mail service encountered an error.
You can also try to use the mail service yourself in API Docs and see if any errors are returned. A successful send should result in a 200 response with content of { count:1 } (I may be slightly off on the formatting.)

Of course you always want to check your spam folder for missed messages as well.