That’s not an issue with a custom domain name, but one of the other parts you run into, SPF and DKIM dns settings being correct and the reputation of whatever SMTP server’s ip address is. No one spam-bins based just on random domain names, or every business would freak out. You can also use your own domain on Google, Microsoft, or Apples ecosystems, not that you need to, there are plenty of providers that will host your email. I like runbox.