Network Load Balancer blocks incoming SMTP traffic?

We need to be able to receive and parse a certain type of notification emails from a 3rd party company. After a bit of experimenting around I decided to spin up a custom SMTP server w/ python and put it on ECS behind a Network Load Balancer. Nothing complex, just tries to parse the email and save data to DB.

I have put together some terraform to create a Network Load Balancer with listener on port 25 and it just hangs as if security group is not configured. Triple checked everything, all as expected.

What's interesting if I use a different listener port on ELB, e.g. 2525, then everything works fine. I could only assume there is some sort blockage on AWS side? Is anyone familiar with the issue?

Again, just to be clear, we are trying to receive emails, we don't need to send them. It's the inbound traffic. So either AWS documentation is incomplete and they also block inbound traffic, or it's something else.