I've been using Fastmail for a few years with my own domain, and it's great. I can't speak to Gmail and if they have this feature, but I use custom aliases with any company or service I do business with (for example, I'll do southwestairlines112026@[domain].net). When the company is hacked and my email address is leaked, or if the company decides to not respect the "Unsubscribe" button, I disable that alias and force it to do a bounce back. This is a feature I only recently discovered and love it.