Hello,
Unfortunately, there's no enough information in your email for anyone to easily help. You will likely need to gather more information around the possible cause.
I'd recommend first looking for error messages in the DSpace backend logs by following the troubleshooting guide:
https://wiki.lyrasis.org/display/DSPACE/Troubleshoot+an+error Try registering a new user and see if any error appears in those logs. You also may need to check the logs of the email system which DSpace is configured to use. It is possible your email system is failing to send or blocking the email for some reason.
Finally, you also could try to test a variety of email addresses. It's possible that only some email addresses are working while others are not (that may imply an issue more at the level of your email system though, as DSpace sends all emails in the same manner).
Good luck and let us know if you find more clues.
Tim