For my custom domain I set up a catch-all so *@sharparam.com gets routed to my main address. If I end up needing to reply from such an address I set up a proper alias for it (currently I use GSuite to manage it).
Another alternative if you don’t have your own domain is to put a plus in the email address. The mail server will ignore anything after that plus, eg johnsmith+facebooklogin@gmail.com will have all email sent to johnsmith@gmail.com, but it will preserve the To header in the email.
This is useful for detecting the origin of spam, however it’s trivial for a spammer or hacker to workaround (just strip the plus and anything after it before sending)
