diff --git a/src/govoplan_mail/backend/sending/imap.py b/src/govoplan_mail/backend/sending/imap.py
index 55e7a57..dc6cb08 100644
--- a/src/govoplan_mail/backend/sending/imap.py
+++ b/src/govoplan_mail/backend/sending/imap.py
@@ -685,7 +685,7 @@ def _parse_fetch_parts_with_sequence(data: list[Any] | tuple[Any, ...] | None) -
 
 
 def _select_readonly(client: imaplib.IMAP4, folder: str) -> tuple[int, str | None]:
-    typ, data = client.select(folder, readonly=True)
+    typ, data = client.select(_quote_mailbox_name(folder), readonly=True)
     if typ != "OK":
         raise ImapAppendError(f"IMAP folder {folder!r} could not be opened read-only: {data!r}", temporary=False)
     selected_count = _decode_item(data[0] if data else None).strip()
@@ -1017,7 +1017,7 @@ def append_message_to_sent(
         client = _open_imap(imap_config)
         target_folder = _effective_sent_folder(config=imap_config, requested_folder=folder, client=client)
         internal_date = imaplib.Time2Internaldate(time.time())
-        typ, data = client.append(target_folder, "\\Seen", internal_date, message_bytes)
+        typ, data = client.append(_quote_mailbox_name(target_folder), "\\Seen", internal_date, message_bytes)
         if typ != "OK":
             raise ImapAppendError(f"IMAP APPEND failed for folder {target_folder!r}: {data!r}", temporary=False)
         response = "; ".join(_decode_item(item) for item in (data or [])) or None
diff --git a/tests/test_imap_parser.py b/tests/test_imap_parser.py
index 152fef5..7c1f849 100644
--- a/tests/test_imap_parser.py
+++ b/tests/test_imap_parser.py
@@ -1,14 +1,18 @@
 from __future__ import annotations
 
 import unittest
+from unittest.mock import patch
 
+from govoplan_mail.backend.config import ImapConfig
 from govoplan_mail.backend.sending.imap import (
     _detect_sent_folder,
     _extract_mailbox_name,
     _normalize_mailbox_page,
     _paged_descending_sequences,
     _parse_fetch_sequence,
+    _select_readonly,
     _sequence_set,
+    append_message_to_sent,
 )
 
 
@@ -69,5 +73,47 @@ class ImapMessagePaginationTests(unittest.TestCase):
         self.assertEqual(_normalize_mailbox_page(folder=" Sent ", limit=500, offset=3), ("Sent", 100, 3))
 
 
+class ImapMailboxCommandTests(unittest.TestCase):
+    def test_select_quotes_mailbox_name_with_spaces(self):
+        class Client:
+            untagged_responses = {"EXISTS": [b"0"], "UIDVALIDITY": [b"1"]}
+
+            def select(self, mailbox, readonly=False):
+                self.mailbox = mailbox
+                self.readonly = readonly
+                return "OK", [b"0"]
+
+            def response(self, code):
+                return "OK", [b"1"] if code == "UIDVALIDITY" else []
+
+        client = Client()
+
+        self.assertEqual(_select_readonly(client, "Gesendete Elemente"), (0, "1"))
+        self.assertEqual(client.mailbox, '"Gesendete Elemente"')
+        self.assertTrue(client.readonly)
+
+    def test_append_quotes_mailbox_name_with_spaces(self):
+        class Client:
+            def append(self, mailbox, flags, date_time, message):
+                self.mailbox = mailbox
+                self.flags = flags
+                self.date_time = date_time
+                self.message = message
+                return "OK", [b"APPEND completed"]
+
+            def logout(self):
+                return "BYE", [b"logged out"]
+
+        client = Client()
+        config = ImapConfig(host="imap.example.org", username="user", password="secret", sent_folder="auto")
+
+        with patch("govoplan_mail.backend.sending.imap._open_imap", return_value=client):
+            result = append_message_to_sent(b"Subject: test\r\n\r\nBody", imap_config=config, folder="Gesendete Elemente")
+
+        self.assertEqual(client.mailbox, '"Gesendete Elemente"')
+        self.assertEqual(result.folder, "Gesendete Elemente")
+        self.assertEqual(result.bytes_appended, len(b"Subject: test\r\n\r\nBody"))
+
+
 if __name__ == "__main__":
     unittest.main()