I’ve used Mimecast to do this for free https://community.mimecast.com/s/article/Stationery-Layouts-Inline-Signatures-1998822414 , there are some limitations

1. Internal Emails do not get stamped with Signatures as these do not go via the Gateway
2. A disclaimer has to be added at the bottom of the email , Mimecast uses this to know if the signature has been applied before or not for replies or forwards
3. Users can’t see the Signature in their Email client to change