I use Fastmail with my own domain and 1Password. Together they give me a “masked email” button for forms that generates a random enough email address (two common words and four digits) and records the domain it was for. You can also create them ad-hoc from Fastmail’s interface.

As well as simply attributing leaks, it’s most valuable as a phishing filter. Why would my bank ever email an address I only used to trial dog food delivery?

Yeah, Fastmail's aliases are great. I used to do things described by some other commenters, like myemail+nameofservice@ and whatnot, but this way the email is automatically generated and you don't have to put any thought into it.