Postfix error - Relay access denied when sending from 'local' account. Config advice please :)

OK my journey of discovery continues quite merrily but I’ve hit a snag with Postfix under Virtualmin.
Created the virtual server, created user/email account, went into Outlook on my local machine at home and set up the account, Outlook seemed happy with the settings … even congratulated me :wink: Receiving mail is working but when I send from my account in Outlook, using, for SMTP,, I get a relay access denied. Now, I understand that out of the box, Postfix will only deliver mail sent from its local subnet?? (Could have that completely round my neck). So what am I missing? Can I tell Postfix, hey, these accounts are your friends, look, they’re even sitting on the same IP as you, it’s OK, you can trust them… have one of these pills…there now.

Any suggestions appreciated.

Afterthought - just been reading this
where it seems to suggest I need to enable or allow specific networks or IP ranges, from which Postfix will relay?? How does that work with a mobile client whose IP address will change?

I think I might be missing something because the localhost directive in the conf file would seem to cover accounts on the Postfix server’s box?? Hmm help needed…

Afterafterthought : my relayhost parameter is empty (the default). Relevant?

OK - I added the external public facing address of my router to the mynetworks list and the mail goes out. I am sure I don’t have to specify all the IPs from which I might ever need to send email via this account…?