> How can I refuse mail from hosts who don't have an open port 25?
>
> What do you think from such a check?

i have tried this, it's not useful, so i didn't leave the check in
place

it's very common, perhaps even the norm that the IP address which
delivers mail to me itself will not accept an incoming port 25
connection

> I've investigated why somebody did not receive mail from a virtual
> machine, and I found out her provider (reviced.nl) refuses all mail
> from a host what does not have port 25 open. I have much problems
> with spam and I would like to reduce it.

this will stop a lot of legitimate mail

and probably not stop much spam