"Important Notice Regarding Your Domain Name(s)" email is a legitimate email from NityGity.com and is simply a reminder to make sure your contact info is up-to-date. Such notifications are sent from the <donotreply@name-services.com> email, which is also legitimate.
If the contact information for your domain(s) needs to be updated, please do so from your NityGity account.
Such emails are required by ICANN. You can also check more information about this here.