While I'm doing web development I want to have an email server that ensures that all mail will end up in my mailbox. To this end I set up Postfix with this rule in main.cf:

header_checks = regexp:/etc/postfix/header_checks.regexp

The header_checks.regexp then contains:

/^To:/ REDIRECT sybren+keep-local@my.domain

This results in all mail being sent via my SMTP server is delivered to my mailbox, regardless of who it's intended for.