

My ISP offers unauthenticated SMTP for their customer addresses. It’s pretty heavily rate limited, but well enough for the occasional status email my system throws out. At least around here they are pretty common for residential customers, so that might be an option.
Other possibility is to set up SMTP relay to your network and give it credentials to an actual email account (with SMTP support, which isn’t granted these days). It can take in emails from your network without credentials and throw them to the internet with your account. It’s pretty simple to set up, search for postfix + smarthost. Added bonus is that you can read the replies to that address too via your email provider services.

Here’s Finnish guide for this
It’s not specifically for war, but for any disruption the society might have. We’ve had winter storms which cut power for few days, wildfires or floods can leave you stranded for a while, a lot of snow might cut roads and so on. Being generally prepared to be self-sufficient for few days just makes sense. On a bigger scale it also helps rescue workers if everyone isn’t in immediate crisis after 24 hours without working infrastructure.