1) Login to Smartermail by accessing the URL: http://mail.domainname/
2) Go to the last button of the top panel > Settings
3) On the left panel, please click at Signature
4) Click on + New Signature
5) Insert the Signature Name where you need to map it to your email account later. And insert the message in signature and Save it.
6) Once done, go to the Mappings on same page, select the signature that create and save it.