I know there’s a lot of material on this matter over there, but every time I follow them I get lost and don’t find clear explanation. I recently moved from one provider to another. The DNS’s are maintained by the provider and I found that there are some miss configurations and it is taking hard to fix. Meanwhile, because I fear being blacklisted, I want to prevent my mail users, but postmaster, to send mails to off-site destinations and I’d like to send the users an excuse message too. Can some one provide a clear example on how to achieve that? I’ll appreciate any help.