CMS is only supported in IBM Smartcloud when used in a hybrid environment. This configuration means that external emails still pass via the on premises server where CMS will append the signatures.
Server signatures will be applied so long as the emails from the Smart cloud are routed to pass through your on premise server before transferring to the internet. Crossware is able to lookup to your NAB and apply the signature.
All users ---> External users = Signature applied
Smartcloud user ---> On Premise internal user = Signature applied
On Premise user ---> Smart cloud internal user = Signature applied
Smartcloud user ---> Smart cloud internal user = No signature (this is because the email has not passed through the on premise server (routed via the smart cloud servers) and has not been stamped by Crossware)
Please also find additional information in the IBM Redbook: