Adding Domains best practise?

[newmember]

All my domains have mail.domain.com where my MX records point to. When adding domains to Zimbra, should I add domain.com with a virtual host for each domain like mail.domain.com? or should I just add mail.domains.com in the domains? or Should I add domain.com and mail.domain.com as each is own domain?
Its interesting that by default Zimbra added my mail.domain.com and domain.com during install.
Second part: When adding new accounts do they get added as; user@domain.com or user@mail.domain.com?

Thanks

[jholder]

I'd just create a new domain for domain.com along side mail.domain.com