Custom domains that have been fully set up with our service are eligible to create email accounts that end with @yourdomain.com. Our email accounts feature is available with an active deluxe plan subscription.
Temporary website addresses (that end with .communitysite.com) are not eligible for creating email accounts. Some communities may have an active deluxe plan subscription, but no custom domain set up yet.
It's possible that your community has a registered domain with another company and has it set up to redirect visitors to your community's temporary address. This type of setup does not allow for creating custom email accounts on that domain, since the domain isn't actually set up with our service.