Some ISP’s even go further my catching all traffic to port 25 to any system outsuide their network (other than to their own MTAs) blocking it or directing that to their own MTA. That is because of course one hacked system in their network means an open relay from their network and that is bad for the reputation of the network. So, they may force all clients to go to their relay. I guess not many ISPs still do this anymore, though some may still block port 25 to anywhere but their own mail relay.
It is many years ago, but at some point my mail could not be delivered (I was sending to 25 with STARTTLS) and after investigation it turned out that the ISP (in this case telecom provider KPN) was redirecting all traffic to their own MTA which then spoofed being my mail server. It was very funny to try connect to my mail.rna.nl postfix MTA to be greeted with “This is sendmail at mail.rna.nl”. Of course this went wrong as soon as authentication was started. Gerben Wierda Chess and the Art of Enterprise Architecture <https://ea.rna.nl/the-book/> Mastering ArchiMate <https://ea.rna.nl/the-book-edition-iii/> Architecture for Real Enterprises <https://www.infoworld.com/blog/architecture-for-real-enterprises/> at InfoWorld On Slippery Ice <https://eapj.org/on-slippery-ice/> at EAPJ > On 13 Jan 2020, at 01:38, Dan Mahoney <d...@prime.gushi.org> wrote: > > Presumably they know what your IP is because they gave it to you. That’s the > authorization. > > I am willing to bet that if you tried to send mail from off network you > wouldn’t be able to without doing SMTP auth > > Sent from my iPhone > >> On Jan 12, 2020, at 16:15, Wesley Peng <wes.aac...@gmail.com> wrote: >> >> >> Hello >> >> My ISP email even doesn’t require SMTP AUTH. Will they be acting as open >> relay? How to stop abuse of outgoing mail? >> >> Regards >
