Gmail is the only thing I wouldn't be able to replace.
I've read it's almost impossible to run your own email server without getting blocked by all recipients in 2025.
I'm running my email server on some famous VPS provider, and with properly configured server (SPF, DCIM), I didn't really get any problems sending emails.
I don't self-host everything. For example, I use Fastmail with custom domains. I think this spreads the risk.
Unfortunately, Google has a policy of "greylisting" domains it had not yet seen, so this increases friction with people on Google. I'm not even running my own email server :(