Re: Refuse mail from hosts with closed port 25

[Jim Reid]

> On 16 Sep 2019, at 13:47, Paul van der Vlis <p...@vandervlis.nl> wrote:
> 
> How can I refuse mail from hosts who don't have an open port 25?
> 
> What do you think from such a check?

It’s a stunningly bad idea. Don’t do it.

Many enterprises and cloud-based mail providers have discrete servers/systems 
handling inbound and outbound mail. In these setups, the servers sending you 
email won’t have a listener on port 25 -- or any other port -- for inbound 
email.

> Is there more needed?  E.g. a list of exceptions for some big providers?

It’ll be impractical to maintain a workable whitelist. There will probably be 
too many false positives and negatives. And the approach probably won’t be an 
effective anti-spam measure either. But if you want to try the experiment and 
report back, go ahead. It’ll only be you and your customers who will have to 
deal with the consequences.