[ptype]

If it's my own domain (probably not unusual among the HN crowd), I would sign up to e.g. kazinator.com by 'inventing' an email address [e-mail], which is forwarded to my gmail through a catch-all email forwarding setup with my domain registrar. Emails to [e-mail] now arrives to my gmail and hence it's easy to block email (e.g. spam) arriving to this email address as well as knowing who leaked my email address to third parties.

As people have pointed out, it now seems possible to use the + functionality of gmail which I was not aware of, but the above setup avoids the issue of forms not accepting + in a valid email address.

This only works with a catch-all email forwarding as I mentioned, so if you want bounce on non-valid addresses it will not work, but like I said it has served me very well.