[mstibbard]

Fastmail brought out a Masked Email feature last year which does this for you! I'm a big fan.

"Keep your real email address private. We’ll create a new address you can use when subscribing to services online, that sends to your inbox. If you start receiving spam, you can block that address.

New Masked Email addresses will be created @fastmail.com. Want to use your custom domain? You can change this in Settings → Domains."