To set up SPF and DKIM for Mimecast, please follow the steps given below:

SPF:

If you don’t have SPF record and wants to set the SPF record for Mimecast then :

• Log in to your DNS management console
• Navigate to your domain section and publish the following SPF record: 

            v=spf1 include:_netblocks.mimecast.com ~all

If you already have SPF record and wants to add the Mimecast domain into existing SPF record then:

• Log in to your Domain Registrar
• Modify your domain’s SPF record to specify Mimecast as the authorized outbound service

For Example:

If your previous SPF record was 

          v=spf1 include:_spf.google.com ~all 

your new SPF record will be 

          v=spf1 include:_spf.google.com include:_netblocks.mimecast.com ~all

DKIM:

• 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