Improve X-Spam-Flag handling

Cargo.lock

@@ -77,9 +77,9 @@ checksum = "47be2f14c678be2fdcab04ab1171db51b2762ce6f0a8ee87c8dd4a04ed216135"
 
 [[package]]
 name = "libc"
-version = "0.2.86"
+version = "0.2.94"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "b7282d924be3275cec7f6756ff4121987bc6481325397dde6ba3e7802b1a8b1c"
+checksum = "18794a8ad5b29321f790b55d93dfba91e125cb1a9edbd4f8e3150acc771c1a5e"
 
 [[package]]
 name = "milter"
@@ -148,9 +148,9 @@ checksum = "3831453b3449ceb48b6d9c7ad7c96d5ea673e9b470a1dc578c2ce6521230884c"
 
 [[package]]
 name = "proc-macro2"
-version = "1.0.24"
+version = "1.0.26"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "1e0704ee1a7e00d7bb417d0770ea303c1bccbabf0ef1667dae92b5967f5f8a71"
+checksum = "a152013215dca273577e18d2bf00fa862b89b24169fb78c4c95aeb07992c9cec"
 dependencies = [
  "unicode-xid",
 ]
@@ -184,9 +184,9 @@ checksum = "8ea5119cdb4c55b55d432abb513a0429384878c15dde60cc77b1c99de1a95a6a"
 
 [[package]]
 name = "syn"
-version = "1.0.60"
+version = "1.0.70"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "c700597eca8a5a762beb35753ef6b94df201c81cca676604f547495a0d7f0081"
+checksum = "b9505f307c872bab8eb46f77ae357c8eba1fdacead58ee5a850116b1d7f82883"
 dependencies = [
  "proc-macro2",
  "quote",

src/email.rs

@@ -145,6 +145,7 @@ pub static REPORT_HEADERS: Lazy<HeaderSet<'static>> = Lazy::new(|| {
 pub struct HeaderRewriter<'a> {
     original: HeaderMap,
     processed: HeaderSet<'a>,
+    spam: Option<bool>,
     spam_assassin_mods: Vec<HeaderMod<'a>>,
     rewrite_mods: Vec<HeaderMod<'a>>,
     report_mods: Vec<HeaderMod<'a>>,
@@ -156,6 +157,7 @@ impl<'a> HeaderRewriter<'a> {
         Self {
             original,
             processed: HeaderSet::new(),
+            spam: None,
             spam_assassin_mods: vec![],
             rewrite_mods: vec![],
             report_mods: vec![],
@@ -166,6 +168,9 @@ impl<'a> HeaderRewriter<'a> {
     pub fn process_header(&mut self, name: &'a str, value: &'a str) {
         // Assumes that the value is normalised to using CRLF line breaks, and
         // includes leading whitespace.
+        if name.eq_ignore_ascii_case("X-Spam-Flag") {
+            self.spam.get_or_insert_with(|| value.trim().eq_ignore_ascii_case("YES"));
+        }
         if is_spam_assassin_header(name) {
             if let Some(m) = self.convert_to_header_mod(name, value) {
                 self.spam_assassin_mods.push(m);
@@ -199,14 +204,7 @@ impl<'a> HeaderRewriter<'a> {
     }
 
     pub fn is_flagged_spam(&self) -> bool {
-        use HeaderMod::*;
-
-        self.spam_assassin_mods.iter().any(|m| match m {
-            Add { name, value } | Replace { name, value } => {
-                name.eq_ignore_ascii_case("X-Spam-Flag") && value.trim().eq_ignore_ascii_case("YES")
-            }
-            _ => false,
-        })
+        self.spam == Some(true)
     }
 
     pub fn rewrite_spam_assassin_headers(