Document (and enforce) that DKIM-signing is not supported in cobination with cutthrough routing

2 files changed, 21 insertions, 5 deletions

diff --git a/doc/doc-docbook/spec.xfpt b/doc/doc-docbook/spec.xfpt
index 88308ba23..1ba0a10dd 100644
--- a/doc/doc-docbook/spec.xfpt
+++ b/doc/doc-docbook/spec.xfpt
@@ -27295,6 +27295,9 @@ after the ACL completes.
 
 Note that routers are used in verify mode.  Note also that headers cannot be
 modified by any of the post-data ACLs (DATA, MIME and DKIM).
+Cutthrough delivery is not supported via transport-filters or when DKIM signing
+of outgoing messages is done, because it sends data to the ultimate destination
+before the entire message has been received from the source.
 
 Should the ultimate destination system positively accept or reject the mail,
 a corresponding indication is given to the source system and nothing is queued.
@@ -27305,7 +27308,6 @@ line.
 
 Delivery in this mode avoids the generation of a bounce mail to a (possibly faked)
 sender when the destination system is doing content-scan based rejection.
-Cutthrough delivery is not supported via transport-filters.
 
 
 .vitem &*control&~=&~debug/*&<&'options'&>
@@ -36347,7 +36349,9 @@ disabled by setting DISABLE_DKIM=yes in Local/Makefile.
 Exim's DKIM implementation allows to
 .olist
 Sign outgoing messages: This function is implemented in the SMTP transport.
-It can co-exist with all other Exim features, including transport filters.
+It can co-exist with all other Exim features
+(including transport filters)
+except cutthrough delivery.
 .next
 Verify signatures in incoming messages: This is implemented by an additional
 ACL (acl_smtp_dkim), which can be called several times per message, with
@@ -36438,6 +36442,10 @@ used.
 Verification of DKIM signatures in incoming email is implemented via the
 &%acl_smtp_dkim%& ACL. By default, this ACL is called once for each
 syntactically(!) correct signature in the incoming message.
+A missing ACL definition defaults to accept.
+If any ACL call does not acccept, the message is not accepted.
+If a cutthrough delivery was in progress for the message it is
+summarily dropped (having wasted the transmission effort).
 
 To evaluate the signature in the ACL a large number of expansion variables
 containing the signature status and its details are set up during the
diff --git a/src/src/verify.c b/src/src/verify.c
index 911d67227..c103f592f 100644
--- a/src/src/verify.c
+++ b/src/src/verify.c
@@ -698,10 +698,18 @@ else
 
     /* For now, transport_filter by cutthrough-delivery is not supported */
     /* Need proper integration with the proper transport mechanism. */
-    if (cutthrough_delivery && addr->transport->filter_command)
+    if (cutthrough_delivery)
       {
-      cutthrough_delivery= FALSE;
-      HDEBUG(D_acl|D_v) debug_printf("Cutthrough cancelled by presence of transport filter\n");
+      if (addr->transport->filter_command)
+        {
+        cutthrough_delivery= FALSE;
+        HDEBUG(D_acl|D_v) debug_printf("Cutthrough cancelled by presence of transport filter\n");
+        }
+      if (ob->dkim_domain)
+        {
+        cutthrough_delivery= FALSE;
+        HDEBUG(D_acl|D_v) debug_printf("Cutthrough cancelled by presence of DKIM signing\n");
+        }
       }
 
     SEND_FAILED: