Some mail providers (eg Gmail) use the plus sign as a label. So your everything still gets routed to d@nblows.com but tagged "hn" or "twitter" or whatever. It'

Some mail providers (eg Gmail) use the plus sign as a label. So your everything still gets routed to d@nblows.com but tagged "hn" or "twitter" or whatever. It's quite a convenient way to have site specific addresses while still only having a single mailbox to manage.