doc: example-config: Document the new submission_backend_capabilities setting.

diff --git a/doc/example-config/conf.d/20-submission.conf b/doc/example-config/conf.d/20-submission.conf
index 078faf128b02e4f57732f8c2830d4ff25367ce8c..390089c31d3ea6254a138bcf8e61b87955ef21b5 100644 (file)
--- a/doc/example-config/conf.d/20-submission.conf
+++ b/doc/example-config/conf.d/20-submission.conf
@@ -77,6 +77,29 @@
 
 # BURL is configured implicitly by IMAP URLAUTH
 
+# Part of the SMTP capabilities that the submission service can offer to the
+# client (as listed in the EHLO reply) depend on those capabilities also being
+# provided by the relay server. These capabilities currently are:
+#
+#   - 8BITMIME
+#   - BINARYMIME
+#   - DSN
+#   - VRFY (always returns 252 without support)
+#
+# By default, the submission service first connects to the relay server to
+# determine the support for such capabilities before sending the initial EHLO
+# reply to the client. If the list of capabilities returned by the relay server
+# is somehow unreliable or it is undesirable to start the connection to the
+# relay server before the first mail transaction is started, the backend
+# capabilities can be configured explicitly using the
+# submission_backend_capabilities setting. This is a space-separated list of
+# SMTP capability names. This setting is only relevant for capabilities that
+# depend on support from the relay server: including (or omitting) capabilities
+# that are not listed above has no effect. When this setting is explicitly set
+# to the empty string, none of the capabilities is enabled. To achieve the
+# default behavior, this setting must be left unconfigured.
+#submission_backend_capabilities =
+
 protocol submission {
   # Space-separated list of plugins to load (default is global mail_plugins).
   #mail_plugins = $mail_plugins