Oh I think I may have sorted it out.
I installed iRedMail (exactly the same version, I found the out of date message and just blocked the if statement in the bash script) it's ok honestly this won't be available to the public (nor is my live server either, whilst I upgrade it anyway manually).
It's just say I was using mail.mydomain2.co.uk as the POP and IMAP services right in outlook?
I have iRedMail setup (from the install anyways) as mail.mydomain1.co.uk
I see in there that it was handled by mail.mydomain1.co.uk but I want it all to be handled by mail.mydomain2.co.uk are you with me?
I hope this helps explain what I am after, reason being I checked on a good DKIM reflector, that it scored quite low for valid mail as opposed to spam because of this.