You Create a Public Folder in your Tenant, however signatures do not get appended even though it looks correctly configured
Below are the log entries, when sending from a Public Folder:
XWARE.MailSignature.DirectoryFunctions_GraphAPI - IsInternalUser-No userPrincipalName in JSON.
Public Folders even though they are "mail enabled" do not have a "mail" attribute i.e., they need to be a User with a valid userPrincipalName.
Crossware Mail Signature will only cache addresses for users with a "mail" attribute.
This is why Crossware Mail Signature cannot process messages sent from a Public Folder.