When you are working with name service domain names and mail domain names, remember the following.
By default, the sendmail program strips the first component from the NIS or NIS+ domain name to form the mail domain name. For example, if an NIS+ domain name were bldg5.example.com, its mail domain name would be example.com.
Although mail domain addresses are case insensitive, the NIS or NIS+ domain name is not. For the best results, use lowercase characters when setting up the mail and NIS or NIS+ domain names.
The DNS domain name and the mail domain name must be identical.
For more information, refer to Interactions of sendmail With Name Services.