tls_key — TLS key to use for outbound or inbound mail
TLS_Key = "/path/to/key"
TLS_Key specifies the location of the PEM encoded key associated with the certificate if the key is not contained within the certificate file. Both TLS_Certificate and TLS_Key are optional and the default behavior is to not present a client certificate to the remote mail exchange at all.
tls_key is valid in the ecstream_listener, esmtp_listener, global, http_listener, listen, pathway, pathway_group and peer scopes.