Proton Mail with a custom domain. The only reason why is that I had it before I knew Fastmail existed and it would be a pain in the ass to move my entire family to it. However, I was VERY tempted when 1Password put Fastmail temporary email support into their product.

Fortunately, Proton Mail just released their own temporary email product based on SimpleLogin.

Plot Twist: OP works at OpenAI and is asking “for a friend”. :)