I don't know how mimecast works... maybe you can get some help from mimecast? Sorry.
I did manage to figure it out, although I think maybe you somewhat misunderstood my original post, i probably could have explained better, so not your fault 
Essentially how I have it set up is that there are multiple email servers across the country that both send AND receive with the same domain, but each branch has their own set of email addresses not the same across branches. Hence when trying to email other branches then Postfix was was seeing the same domain name and trying to lookup locally instead of directing it to the relayhost.
The mention of Mimecast only relates to how mails are delivered to the correct server (for incoming mails)
My solution was to create a transport rule in Postfix
externaluser@domain.tld dovecot
domain.tld smtp:za-smtp-outbound-1.mimecast.co.za
* :
This way, internal mails will go to the local mailbox, and everything else goes to the relayhost.
Would there be a better way to achieve this?