2 files changed, 13 insertions, 0 deletions

diff --git a/etc/NEWS b/etc/NEWS
index a6fd51b8a2a..11fca4fecb6 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -1138,6 +1138,11 @@ bindings, will be aborted, and Emacs will not ask you whether to
 enlarge 'max-specpdl-size' to complete the rendering.  The default is
 t, which preserves the original behavior.
 
+---
+*** New user option 'rmail-show-message-set-modified'.
+If set non-nil, showing an unseen message will set the Rmail buffer's
+modified flag.
+
 ** Apropos
 
 *** New commands 'apropos-next-symbol' and 'apropos-previous-symbol'.
diff --git a/lisp/mail/rmail.el b/lisp/mail/rmail.el
index 29460cc20f5..9f95b62d870 100644
--- a/lisp/mail/rmail.el
+++ b/lisp/mail/rmail.el
@@ -2723,6 +2723,12 @@ See also `unrmail-mbox-format'."
   :version "24.4"
   :group 'rmail-files)
 
+(defcustom rmail-show-message-set-modified nil
+  "If non-nil, displaying an unseen message marks the Rmail buffer as modified."
+  :type 'boolean
+  :group 'rmail
+  :version "28.1")
+
 (defun rmail-show-message-1 (&optional msg)
   "Show message MSG (default: current message) using `rmail-view-buffer'.
 Return text to display in the minibuffer if MSG is out of
@@ -2750,6 +2756,8 @@ The current mail message becomes the message displayed."
         ;; Mark the message as seen, but preserve buffer modified flag.
         (let ((modiff (buffer-modified-p)))
           (rmail-set-attribute rmail-unseen-attr-index nil)
+          (and rmail-show-message-set-modified
+               (setq modiff t))
           (unless modiff
             (restore-buffer-modified-p modiff)))
         ;; bracket the message in the mail