feat: queue messages for SMTP before encryption

Headers like From and Autocrypt are now added late,
right before sending the message over SMTP.
This way we advertise the latest list of transports
and use the correct From address in the encrypted part
even for messages queued while being offline.

BCC-self recipients are also added late.
For unencrypted messages we only want to send a copy
to the sending address, but we don't know the sending address
when queueing the message.
Adding bcc-self recipients when dequeuing the message
also makes it possible to send copies to updated list of relays.

deltachat-rpc-client/tests/test_multitransport.py

@@ -98,6 +98,25 @@ def test_change_address(acf) -> None:
     assert sender_addr2 == new_alice_addr
 
 
+def test_remove_transport_keep_messages(acf) -> None:
+    """Test that deleting current sending transport keeps queued messages."""
+    alice, bob = acf.get_online_accounts(2)
+
+    qr = acf.get_account_qr()
+    alice.add_transport_from_qr(qr)
+
+    alice.stop_io()
+    alice_chat_bob = alice.create_chat(bob)
+    alice_chat_bob.send_text("Hello!")
+
+    new_alice_addr = alice.list_transports()[1]["addr"]
+    alice.delete_transport(alice.list_transports()[0]["addr"])
+    alice.start_io()
+
+    bob_msg = bob.wait_for_incoming_msg().get_snapshot()
+    assert bob_msg.sender.get_snapshot().address == new_alice_addr
+
+
 def test_download_on_demand(acf, rpcdata) -> None:
     alice, bob = acf.get_online_accounts(2)
     alice.set_config("download_limit", "1")