iulian.cristea
There are some clues/suggestions re PTR & SPF here
http://wiki.contribs.org/SME_Server:Documentation:Administration_Manual:AppendixThese are really something your ISP does for you, or you do in your external DNS records if you know how to do it, clue - search google.
These are not settings you make on sme server.
To work around the need to configure these settings, then you send mail via your ISP's mail server (set this in server manager email panel), but as you say the option to use your ISP's mail server is not available, then you have no choice but to get records configured by your ISP.
Yahoo (& many others) got tough on spammers over 2 years ago !