diff --git a/lib/mu-query-threads.cc b/lib/mu-query-threads.cc
index fa1bb672..c71651cf 100644
--- a/lib/mu-query-threads.cc
+++ b/lib/mu-query-threads.cc
@@ -366,6 +366,20 @@ to_string (const ThreadPath& tpath, size_t digits)
         return str;
 }
 
+static bool // compare subjects, ignore anything before ':'
+subject_matches (const std::string& sub1, const std::string& sub2)
+{
+	auto search_str =[](const std::string&s) -> const char* {
+		const auto pos = s.find_first_of(':');
+		if (pos == std::string::npos)
+			return s.c_str();
+		else
+			return s.c_str() + pos + 1;
+	};
+
+	//g_debug ("'%s' '%s'", search_str(sub1), search_str(sub2));
+	return g_strcmp0(search_str(sub1), search_str(sub2)) == 0;
+}
 
 static bool
 update_container (Container& container, bool descending,
@@ -385,7 +399,6 @@ update_container (Container& container, bool descending,
                 return false; // nothing else to do.
 
         auto& qmatch(*container.query_match);
-
         if (!container.parent)
                 qmatch.flags |= QueryMatch::Flags::Root;
         else if (!container.parent->query_match)
@@ -394,20 +407,10 @@ update_container (Container& container, bool descending,
         if (!container.children.empty())
                 qmatch.flags |= QueryMatch::Flags::HasChild;
 
-        // see whether this message has the thread subject, i.e., the
-        // first message in this thread with the given subject.
-        if (qmatch.has_flag(QueryMatch::Flags::Root) ||
-            //qmatch.has_flag(QueryMatch::Flags::Orphan) ||
-            prev_subject.empty() ||
-            (qmatch.subject.find(prev_subject) > 5))
+        if (qmatch.has_flag(QueryMatch::Flags::Root) || prev_subject.empty() ||
+	    !subject_matches(prev_subject, qmatch.subject))
                 qmatch.flags |= QueryMatch::Flags::ThreadSubject;
 
-        // g_debug ("%c%c: '%s' vs '%s'",
-        //          any_of(qmatch.flags & QueryMatch::Flags::Root) ? 'r' : 'c',
-        //          any_of(qmatch.flags & QueryMatch::Flags::ThreadSubject) ? 'y' : 'n',
-        //          qmatch.subject.c_str(),
-        //          prev_subject.c_str());
-
         if (descending && container.parent) {
                 // trick xapian by giving it "inverse" sorting key so our
                 // ascending-date sorted threads stay in that order
@@ -427,20 +430,19 @@ update_container (Container& container, bool descending,
 
 static void
 update_containers (Containers& children, bool descending, ThreadPath& tpath,
-                   size_t seg_size, const std::string& prev_subject)
+                   size_t seg_size, std::string& prev_subject)
 {
         size_t idx{0};
-        std::string last_subject = prev_subject;
 
         for (auto&& c: children) {
                 tpath.emplace_back(idx++);
                 if (c->query_match) {
 			update_container(*c, descending, tpath, seg_size,
-                                         last_subject);
-                        last_subject = c->query_match->subject;
+                                         prev_subject);
+			prev_subject = c->query_match->subject;
                 }
                 update_containers(c->children, descending, tpath, seg_size,
-				  last_subject);
+				  prev_subject);
                 tpath.pop_back();
         }
 }