One or more domains can be set up to the primary domain so that each domain shows the same web presentation.
E.g. mydomain.cz = myotherdomain.com = mydifferentotherdomain.eu
Alternatively (or possibly simultaneously) domains can be set up so that all mailboxes receive mails for each domain.
E.g. firstname.lastname@example.org = email@example.com =
Domains, that are set up as alias, have to be registered and have to have same DNS record. Aliases can be order in sets of five.