I switched to Fastmail with my own domain (max.engineer), and as I'm changing email on various services, I'm running into the same issues. Synchrony bank and tr

I switched to Fastmail with my own domain (max.engineer), and as I'm changing email on various services, I'm running into the same issues. Synchrony bank and treasurydirect.gov are among the offenders. My solution for these sites was to use another domain I own (a .com) mapped to the same Fastmail account. My preferred email validation is a combination of string contains '@', actually sending a verification email and/or making an MX lookup.