I have now got postfix up and running and selected my hosted domain to deliver to it. This appears to be working.
I used to collect my hosted mail from another linux server running fetchmail (using my +net account, and this clearly dealt with the hosted domain too). What I have noticed, now, though is that if fetchmail gets to the mail before it is forwarded to my mail server it still collects the hosted domain as well as +net domain.
Is this by design? ie that not only does autoturn keep the message safe if the home email server is down, but also allows pop3 collection as well?