Say you have example.com. You add two email accounts for example.com, John and Haley. Now you have two email accounts setup under example.com, john@example.com and haley@example.com. Months go by and you think example.com is just to long for people to type, so you register ex.com as a replacement/alias for example.com You also don't want to create two more email accounts just for ex.com, you want to be able to check one account that receives both domains for john and haley. Instead of creating an account john@ex.com and set it to forward to your already existing account john@example.com, you go into your mail domain for example.com and add ex.com as a mail domain alias. What this does is allows all accounts that are under @example.com to also be accepted as @ex.com. So there is only one account, john@example.com, but it accepts mail as john@example.com or john@ex.com.
