I'm having to leave Gmail soon (kicking and screaming) because they've made it so you can't send mail using third-party email addresses anymore. I have 8 email

I'm having to leave Gmail soon (kicking and screaming) because they've made it so you can't send mail using third-party email addresses anymore. I have 8 email addresses configured through Gmail, only a few of which I even own the domain for. My main email address goes back 25 years. I've been using Gmail for over a decade, but I guess I have to switch to Fastmail, which is so frustrating because everyone has standardized on Gmail.