diff --git a/runtime/ftplugin/sieve.vim b/runtime/ftplugin/sieve.vim
index 8161fe99ac..1ea8eaeb9d 100644
--- a/runtime/ftplugin/sieve.vim
+++ b/runtime/ftplugin/sieve.vim
@@ -3,6 +3,7 @@
 " Maintainer:           This runtime file is looking for a new maintainer.
 " Previous Maintainer:  Nikolai Weibull <now@bitwi.se>
 " Latest Revision:      2025 Feb 20
+" 2026 Jan 09 by Vim Project: preserve line endings for existing files #19144
 
 if exists("b:did_ftplugin")
   finish
@@ -16,4 +17,7 @@ setlocal formatoptions-=t formatoptions+=croql
 
 " https://datatracker.ietf.org/doc/html/rfc5228#section-2.2 says
 " "newlines (CRLF, never just CR or LF)"
-setlocal fileformat=dos
+" Use CRLF for new files only; preserve existing line endings
+if expand('%:p') !=# '' && !filereadable(expand('%:p'))
+  setlocal fileformat=dos
+endif