[garaetjjte]

Why? I expect from ISPs to deliver IP packets to/from my address, without filtering on basis what is in payload.

Though once when device connected to my wifi got infected and started sending spam, I got angry (not e)mail from ISP, so I drop tcp/25 on my router firewall.

[kalleboo]

So you, someone technical enough to set up their own email server, was spamming people, and it presumably took days or weeks for someone to report you, and you to check your mailbox and get around configuring your firewall.

Now imagine the typical user who has no idea what the letter means or how to configure their router and just ignores it...

I'm surprised your whole ISPs dynamic IP pool isn't already on every spam block list.

edit: just realized you aren't the poster I was replying to, so presumably you're not running your own email server