Customer order mail error

[Vanlombardi]

Some of my customer are not receiving emails after placing and order and shop admins recieve failure mails.

Mail log is giving me the following error:
stat=Deferred: 450 4.7.1 Client host rejected: cannot find your reverse hostname

 

But google users and some domain emails (i tested with mine) are working properly.

Any one had the same issue? I read about dns records, but if some customer can recieve these emails I don't think my server is misconfigured.

Thank you!