There are two types of mail domains. Read here to understand the difference.
Before you’ll be able to add a mail domain you need to create at least one website.
To add mail domain:
Login at Doka panel. in "User menu" navigate to "User -> Managed products -> Email -> Add new mail domain”.
Select Web domain (website name) which you want to convert to mail domain.
If you want to manage mail domain with Doka control panel click “Add local mail domain” hyperlink.
If you want to redirect mail to preset providers (Google Apps) click “Create mail redirect to a partner service”
If you want to redirect mail to another Third party mail provider click “Mail redirect with custom MX records”
Warning! Whether you use custom or preset redirect to a third party mail provider, your mail setup isn’t finished yet! You have to continue the mail setup with the provider you are redirected to. With preset providers, a web link will be shown where you'll need to continue mail setup. With mail providers such as “Google Apps” you’ll need to add the mail domain, pass verification and then add mailboxes. All support also will be provided by the third party mail provider!
The difference between preset Redirect and Custom redirect is that with preset redirect the system adds all required MX records. However, no setup is made on behalf of Mail partner. You’ll need to visit the mail provider and continue to setup the mail in the way you’d do it with custom redirect!