If you are not using LinkU's name servers for your domain name, you will need to setup your DNS records to point to your e-mail to us.
You can usually make these changes from there as well. This is usually referred to Advanced DNS Management, Manage DNS Records, or any similar terminology.
MX records
mx.linkumail.com
Priority 10
mx2.linkumail.com
Priority 20
mx3.linkumail.com
Priority 30
E-mail CNAME (optional):
mail.yourdomaingoeshere.com -> pop.linkumail.com