diff --git a/mumi/client.scm b/mumi/client.scm
index c30429d..b89e608 100644
--- a/mumi/client.scm
+++ b/mumi/client.scm
@@ -17,6 +17,7 @@
 ;;; along with mumi.  If not, see <http://www.gnu.org/licenses/>.
 
 (define-module (mumi client)
+  #:use-module (rnrs exceptions)
   #:use-module (rnrs io ports)
   #:use-module (srfi srfi-1)
   #:use-module (srfi srfi-19)
@@ -38,7 +39,9 @@
             print-current-issue
             set-current-issue!
             clear-current-issue!
-            send-email))
+            send-email
+            git-send-email-headers
+            compose))
 
 (define (git-top-level)
   "Return the top-level directory of the current git repository."
@@ -229,6 +232,13 @@ arguments."
                      name)
                  " <" address ">"))
 
+(define (split-cc cc)
+  "Split CC into a list of email addresses."
+  (map (lambda (address)
+         (serialize-email-address (assq-ref address 'name)
+                                  (assq-ref address 'address)))
+       (assq-ref (parse-email-headers (string-append "Cc: " cc "\n"))
+                 'cc)))
 
 (define* (git-send-email to patches #:optional (options '()))
   "Send PATCHES using git send-email to the TO address with
@@ -311,3 +321,43 @@ ISSUE-NUMBER."
                          "@"
                          (client-config 'debbugs-host))
           other-patches)))))
+
+(define (git-send-email-headers patch)
+  "Print send-email headers for PATCH."
+  (let* (;; Compute headers if configured in git config.
+         (header-command
+          (guard (ex (#t #f))
+            (call-with-input-pipe* (list "git" "config" "sendemail.headerCmd")
+              get-line)))
+         (headers
+          (if header-command
+              (call-with-input-pipe (string-append header-command " " patch)
+                get-string-all)
+              ""))
+         (external-x-debbugs-cc
+          (cond
+           ((assq-ref (parse-email-headers (string-append headers "\n"))
+                       'x-debbugs-cc)
+            => split-cc)
+           (else '())))
+         ;; Fetch Cc addresses for current issue.
+         (x-debbugs-cc
+          (cond
+           ((assq-ref (reply-email-headers (current-issue-number))
+                       'cc)
+            => split-cc)
+           (else '()))))
+    ;; Print X-Debbugs-Cc header.
+    (display "X-Debbugs-Cc: ")
+    (display (string-join (delete-duplicates
+                           (append x-debbugs-cc external-x-debbugs-cc))
+                          ", "))
+    (newline)
+    ;; Print headers other than X-Debbugs-Cc.
+    ;; TODO: RFC5322 headers are not restricted to a single
+    ;; line. "Folded" multi-line headers are allowed. Support them.
+    (for-each (lambda (line)
+                (unless (string-prefix-ci? "X-Debbugs-Cc:" line)
+                  (display line)
+                  (newline)))
+              (string-split headers #\newline))))
diff --git a/scripts/mumi.in b/scripts/mumi.in
index 2295328..8fb7cd4 100644
--- a/scripts/mumi.in
+++ b/scripts/mumi.in
@@ -163,6 +163,8 @@
    (client:clear-current-issue!))
   (("send-email" . patches)
    (client:send-email patches))
+  (("git-send-email-headers" patch)
+   (client:git-send-email-headers patch))
   (("mailer" . rest)
    (let* ((opts (parse-options rest))
           (sender (assoc-ref opts 'sender))
diff --git a/tests/client.scm b/tests/client.scm
index 2b2c1be..ced573b 100644
--- a/tests/client.scm
+++ b/tests/client.scm
@@ -68,6 +68,9 @@ called with."
 (define serialize-email-address
   (@@ (mumi client) serialize-email-address))
 
+(define split-cc
+  (@@ (mumi client) split-cc))
+
 (test-begin "client")
 
 (test-equal "serialize email address"
@@ -78,6 +81,11 @@ called with."
   "\"Bar, Foo\" <foobar@example.com>"
   (serialize-email-address "Bar, Foo" "foobar@example.com"))
 
+(test-equal "split Cc field"
+  (list "Foo <foo@example.com>"
+        "\"Bar, Foo\" <foobar@example.com>")
+  (split-cc "Foo <foo@example.com>, \"Bar, Foo\" <foobar@example.com>"))
+
 (test-equal "send patches to new issue"
   '(("git" "send-email" "--to=foo@patches.com" "foo.patch")
     ("git" "send-email" "--to=12345@example.com" "bar.patch" "foobar.patch"))
