Yep, postfix can write to it,
Thats probably not the issue as im getting messages sent to the account saying
This is the mail system at host jacknet.co.uk.
I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.
For further assistance, please send mail to postmaster.
If you do so, please include this problem report. You can
delete your own text from the attached returned message.
The mail system
<email@example.com>: unknown user: "jack"
Mar 15 17:20:49 Breathing postfix/trivial-rewrite: warning: do not list domain jacknet.co.uk in BOTH mydestination and virtual_mailbox_domains