Email is a decentralized protocol and the fact that you use a centralized provider such as gmail only somewhat defeats the purpose of email being a decentralized protocol instead of Google Messenger.
The email protocol isn't complicated though. Back before spam was a thing, lots of people could just open a port and manually talk SMTP (you could send messages as Santa, or whatever). The only service Gmail provides that's not trivial (and I mean, something that can be done in a whiteboard interview) is the spam protection.
A more advanced classifier certainly do better these days.