Yes, with email domain aliases, one domain can receive incoming mail for one or more other domains. For example, if greatmailcorp.com is configured as an email domain alias of greatmail.com, then any email addressed to user@greatmailcorp.com will be delivered to an account user@greatmail.com.
If you have one or more domains that you would like to alias to a single domain, you will need to contact Greatmail support to make this configuration. Both the aliased and primary domains will need to have their MX records pointed to Greatmail for aliasing to work.