diff --git a/tests/imap_fixture.py b/tests/imap_fixture.py
index e26b4a8..b4247bb 100644
--- a/tests/imap_fixture.py
+++ b/tests/imap_fixture.py
@@ -79,8 +79,10 @@ def late():
     a = old("alice", "alicepass")
     append(a, "INBOX", msg("late@test", "arrived after the bulk copy"))
     a.select("INBOX")
-    typ, data = a.search(None, "HEADER", "Message-ID", "<m2@test>")
-    a.store(data[0].split()[0], "+FLAGS", r"(\Seen)")
+    # Find m2 by its header (Stalwart's HEADER search is full-text, not substring).
+    typ, data = a.fetch("1:*", "(BODY.PEEK[HEADER.FIELDS (MESSAGE-ID)])")
+    seq = next(item[0].split()[0] for item in data if isinstance(item, tuple) and b"<m2@test>" in item[1])
+    a.store(seq, "+FLAGS", r"(\Seen)")
     a.logout()