I’m currently using the (very helpful) external DNS help to run mailinabox from my current DNS server. It works a treat. I’ve decided that I want to use mailinabox to handle all mail sending now as this simplifies the DKIM stuff (my other boxes have a local postfix they can use but email going out of there won’t have the right signatures now).
Whats the best practice for getting email to pipe through my mailinabox?
I would say to create a user account and then give the other servers the email/password credentials.
Would I need to create an account for each address that I wish to send emails for? Can an account send emails for any address after the SMTP transaction is authorised?
It’s all mostly just PHP apps and ‘webmaster@’ will likely do but I imagine there will be others needed.
There’s no restriction that the account send mail as any particular address. You can use one set of credentials.
Having a frustrating experience of this so far 
I have
relayhost = [box.example.com]
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/relay_passwd
smtp_sasl_security_options = noanonymous, noplaintext
smtp_sasl_tls_security_options = no anonymous
On my server and it is relaying mail as I would have hoped. However it seems to be refusing to authenticate when relaying since any email not hosted on the box is being bounced as a refused relay. I realise this is a tad out of the scope of mailinabox but is there something I should be looking at to help debug this? Looking at the syslog and mail.logs on both servers is telling me nothing extra.
EDIT
This may be fixed. I amended the above config to
relayhost = [box.example.com]:587
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/relay_passwd
smtp_sasl_security_options = noanonymous, noplaintext
smtp_sasl_tls_security_options = noanonymous
smtp_use_tls = yes
With the important bit being the smtp_use_tls part and now it seems to be relaying ok.