Strip DKIM-Signature header before LMTP

Currently we strip the DKIM-Signature header in the OpenDKIM final.lua
script after validation of the signature. We sign all messages upon
submission, but we do not verify messages which are from a local account
and delivered to another local account.

This corrects the problem and ensures that the plaintext headers of a
local to local delivery are sanitized the same as a message received
from another server.

The functionality in final.lua to strip the DKIM-Signature header can
now be retired.

cmdeploy/src/cmdeploy/postfix/deployer.py

@@ -52,6 +52,15 @@ class PostfixDeployer(Deployer):
         )
         need_restart |= header_cleanup.changed
 
+        lmtp_header_cleanup = files.put(
+            src=get_resource("postfix/lmtp_header_cleanup"),
+            dest="/etc/postfix/lmtp_header_cleanup",
+            user="root",
+            group="root",
+            mode="644",
+        )
+        need_restart |= lmtp_header_cleanup.changed
+
         # Login map that 1:1 maps email address to login.
         login_map = files.put(
             src=get_resource("postfix/login_map"),

cmdeploy/src/cmdeploy/postfix/lmtp_header_cleanup

@@ -0,0 +1 @@
+/^DKIM-Signature:/            IGNORE

cmdeploy/src/cmdeploy/postfix/main.cf.j2

@@ -77,6 +77,7 @@ inet_protocols = all
 
 virtual_transport = lmtp:unix:private/dovecot-lmtp
 virtual_mailbox_domains = {{ config.mail_domain }}
+lmtp_header_checks = regexp:/etc/postfix/lmtp_header_cleanup
 
 mua_client_restrictions = permit_sasl_authenticated, reject
 mua_sender_restrictions = reject_sender_login_mismatch, permit_sasl_authenticated, reject