Login to Mimecast

Select Administration console

From the top left select Administration > Gateway > policies

Select definitions drop down select DNS Authentication - Outbound

Select New DNS Authentication - Outbound signing

Set a name for the definition

Tick the checkbox Sign outbound mail with DKIM

Use the lookup option to select domain to DKIM sign

Select generate

Create a TXT record within your DNS with the public key provided save and exit

Verify public key in Mimecast console

Login to Mimecast

Select Administration Console

From the top left select Administration > Gateway > policies

Search for DNS authentication outbound

Select New Policy

Name the policy

Under emails from use address based on "Both"

Save and Exit