[merb]

It's somewhat true. DKIM checks the authenticity of a email domain. So it definitly helps. SPF does barely the same. that's why you got into spam if you not set RDNS OR DKIM OR SPF. Since the other server can't be sure if the server is allowed to send mails with the provided domain.

Mailservers are simple, basically you can send with every domain available, however that won't work since other servers will handle that via SPF, RDNS or DKIM.