We currently send email from our exchange server, to our mcafee scm 3100 then onto mimecast. We now need to temporarily setup a relay to allow 'DomainA' to send directly to the internet as well as keep our existing relay * (all domains) to mimecast rule.
If I'm not mistaken or misunderstood you, you need a new relay rule above the existing rule so it is evaluated first. The rule applies to DomainA (Message source) and delivery method is DNS delivery. All messages that fail this condition would be evaluated by the second - existing - rule.