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--- dotrc:exim:secondary_mx [2011/09/01 13:41] – created adi
+++ dotrc:exim:secondary_mx [2018/07/10 09:49] (current) – exim: secondary mx -- callout to primary adi
@@ Line 14: / Line 14: @@
 +domainlist relay_to_domains = MAIN_RELAY_TO_DOMAINS : dsearch;CONFDIR/secondary
 </code>
+====== Variant: callout to primary mx ======
+To avoid spam processing and back-scatter the secondary mx may reject mails (temporary error) as long as the primary mail server is online. Add the following to acl/30_exim4-config_check_rcpt (right after the CHECK_RCPT_LOCAL_ACL_FILE for example):
+<code>
+  ## CALLOUT FOR SECONDARY MX
+  ## http://blog.aptivate.org/en/blog/2009/01/28/backup-mail-exchangers/
+  # if we know that the primary MX rejects this address, we should too
+  deny
+    domains = dsearch;MAILCONF/secondary
+    ! verify = recipient/callout=30s,defer_ok
+    message = Rejected by primary MX
+  # detect whether the callout is failing, without causing it to
+  # defer the message. only a warn verb can do this.
+  warn
+    domains = dsearch;MAILCONF/secondary
+    set acl_m_callout_deferred = true
+    verify = recipient/callout=30s
+    set acl_m_callout_deferred = false
+  # if the callout did not fail, and the primary mail server is not
+  # refusing  mail for this address, then it's accepting it, so tell
+  # our client to try again later
+  defer
+    domains = dsearch;MAILCONF/secondary
+    ! condition = $acl_m_callout_deferred
+    message = The primary MX is working, please use it
+  # callout is failing, main server must be failing,
+  # accept everything
+  accept
+    domains = dsearch;MAILCONF/secondary
+    message = Accepting mail on behalf of primary MX
+  ## END CALLOUT FOR SECONDARY
+</code>