smtpsrv: Implement a post-DATA hook

This patch implements a post-DATA hook, which is run after receiving the
data but before sending a reply.

It can be used to implement content filtering when receiving email, for
example for passing the email through an anti-spam or an anti-virus.

hooks/post-data

@@ -0,0 +1,35 @@
+#!/bin/bash
+#
+# This file is an example post-data hook that will run standard filtering
+# utilities if they are available.
+#
+#  - spamc (from Spamassassin) to filter spam.
+#  - clamdscan (from ClamAV) to filter virus.
+
+set -e
+
+TF="$(mktemp --tmpdir "post-data-XXXXXXXXXX")"
+trap 'rm "$TF"' EXIT
+
+# Save the message to the temporary file, so we can pass it on to the various
+# filters.
+cat > "$TF"
+
+
+if command -v spamc >/dev/null; then
+        if ! SL=$(spamc -c - < "$TF") ; then
+                echo "spam detected"
+                exit 1
+        fi
+        echo "X-Spam-Score: $SL"
+fi
+
+
+if command -v clamdscan >/dev/null; then
+        if ! clamdscan --no-summary --infected - < "$TF" 1>&2 ; then
+                echo "virus detected"
+                exit 1
+        fi
+        echo "X-Virus-Scanned: pass"
+fi
+