Send/forward email to email@example.com or firstname.lastname@example.org or <whatever time length>@followupthen.com and it will be sent back to you in that time. Try sending an email to email@example.com to try it. No need to create an account.
I love this service – I can forward emails to firstname.lastname@example.org (or whatever time) and delete/file the email to get it out of my inbox. It’s one of my current favorite finds.
There’s no account creation (unless you want to for advanced features).