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Indeed, however if you're running your own server(s) then firewalls/monitoring/IDS/etc... need to be part of your typical setup anyhow. I wouldn't suggest someone who doesn't admin their own servers to get a server for running mail on, but if you manage/secure/backup servers for other stuff, mail is pretty easy to do.

I self-host my mail too, because I've been doing it for years and I really like having absolute control over what ends up in my Inbox.

True, it is probably easier for someone to break into my Linode than a Google server. However I think it's probably even easier for someone to break into my Google account. The surface area of a Google account is huge nowadays..

automated attacks are easier to prevent, whereas dedicated attacks are harder, and google are much more likely to be the victim of a dedicated attack

Google also has a whole team of people whose only job is to stop attacks.