Originally Posted by
miklcct
It looks like a domestic flight, feels like a domestic flight and behaves like a domestic flight, therefore it is a domestic flight.
The currency changes from markka to riksdaler, the language changes from Finnish and Swedish to only Swedish, you get an SMS welcoming you to a new country (unless you are returning home) and you pass through customs, although it may at times be difficult to find a customs inspector if you have goods to declare. Last year when I was waiting for the ferry from Kapellskär to Mariehamn, I saw someone driving off the ferry, parking in the red "goods to declare" lane and then spending a lot of time trying to figure out how to find a customs inspector. I'm not sure if they were ever successful.