The options for choosing which servers to install on are:
1. If signatures are only required for External email install Crossware Mail signature on the outgoing SMTP server
2. If signatures are required for both Internal and External emails you need to install on all mail servers including the outgoing SMTP servers
3. For local signatures any server can be used even one without mail as the server is only used to push the signatures to users mail files residing on other servers
We have attached a simple diagram showing the options: