Add docs for how to create a smtp gateway

tools/exim/exim.conf

@@ -0,0 +1,57 @@
+## Provide certificates to enable StartTLS
+# tls_certificate = /var/lib/acme/test.org/fullchain.pem
+# tls_privatekey = /var/lib/acme/test.org/key.pem
+tls_advertise_hosts =
+
+primary_hostname = test.org
+domainlist local_domains = test.org
+timeout_frozen_after = 1m
+acl_smtp_rcpt = acl_check_rcpt
+acl_smtp_data = acl_check_data
+never_users = root
+host_lookup = *
+daemon_smtp_ports = 25
+
+message_size_limit = 30m
+
+keep_environment = DS_HEADER : DS_URL
+
+begin acl
+acl_check_rcpt:
+require
+  domains = +local_domains
+require
+  message = Sender verification failed
+  verify = sender
+require
+  message = Receiver verification failed
+  verify = recipient
+require
+  message = Recipient unknown
+  condition = ${run{/usr/bin/curl --out /dev/null --silent --fail -H "Docspell-Integration: ${env{DS_HEADER}{$value} fail}" "${env{DS_URL}{$value} fail}/api/v1/open/integration/item/$local_part"}{yes}{no}}
+warn
+  message = Reverse lookup failed
+  !verify = reverse_host_lookup
+accept
+
+acl_check_data:
+deny
+  message = Sender verification failed
+  !verify = header_sender
+accept
+
+begin routers
+local_users:
+  driver = accept
+  transport = docspell
+
+begin transports
+docspell:
+  driver = pipe
+  command = /usr/bin/curl --out /dev/null --silent --fail -H "Docspell-Integration: ${env{DS_HEADER}{$value} fail}" -F "file=@-;filename=\"$h_subject:\"" "${env{DS_URL}{$value} fail}/api/v1/open/integration/item/$local_part"
+  return_fail_output
+  user = nobody
+  delivery_date_add
+  envelope_to_add
+  return_path_add
+  log_output