I have been using Fastmail for about 3 years, but recently a few of their SMTP server IP seem have been blacklisted.
My email is being rejected left and right. Not sure where to move to
I am renting a SoYouStart [1] server with a self hosted solution. If you're not that experienced with setting up and maintaining an email server, you can try installing the MailCow [2]. Its fairly easy to setup and maintain. There's enough setup guides available, including setting up your DMARC and DKIM so your emails won't be rejected or treated as spam by default.
[0] https://connect.yandex.com/