only send \NonExistent for IMAP4rev2, and automatically subscribe to imported mailboxes

http/import.go

@@ -444,8 +444,11 @@ func importMessages(ctx context.Context, log *mlog.Log, token string, acc *store
 				err = tx.Insert(&mb)
 				ximportcheckf(err, "inserting mailbox in database")
 
-				changes = append(changes, store.ChangeAddMailbox{Name: p})
+				err = tx.Insert(&store.Subscription{Name: p})
-				// todo: should we also subscribe to the mailbox?
+				if err != nil && !errors.Is(err, bstore.ErrUnique) {
+					ximportcheckf(err, "subscribing to imported mailbox")
+				}
+				changes = append(changes, store.ChangeAddMailbox{Name: p, Flags: []string{`\Subscribed`}})
 			} else if err != nil {
 				ximportcheckf(err, "creating mailbox %s (aborting)", p)
 			}

imapserver/delete_test.go

@@ -13,8 +13,12 @@ func TestDelete(t *testing.T) {
 	tc2 := startNoSwitchboard(t)
 	defer tc2.close()
 
+	tc3 := startNoSwitchboard(t)
+	defer tc3.close()
+
 	tc.client.Login("mjl@mox.example", "testtest")
 	tc2.client.Login("mjl@mox.example", "testtest")
+	tc3.client.Login("mjl@mox.example", "testtest")
 
 	tc.transactf("bad", "delete")              // Missing mailbox.
 	tc.transactf("no", "delete inbox")         // Cannot delete inbox.
@@ -26,14 +30,18 @@ func TestDelete(t *testing.T) {
 
 	tc.client.Create("a/b")
 	tc2.transactf("ok", "noop") // Drain changes.
+	tc3.transactf("ok", "noop")
 
 	// ../rfc/9051:2000
 	tc.transactf("no", "delete a") // Still has child.
 	tc.xcode("HASCHILDREN")
 
+	tc3.client.Enable("IMAP4rev2") // For \NonExistent support.
 	tc.transactf("ok", "delete a/b")
 	tc2.transactf("ok", "noop")
-	tc2.xuntagged(imapclient.UntaggedList{Flags: []string{`\NonExistent`}, Separator: '/', Mailbox: "a/b"})
+	tc2.xuntagged() // No IMAP4rev2, no \NonExistent.
+	tc3.transactf("ok", "noop")
+	tc3.xuntagged(imapclient.UntaggedList{Flags: []string{`\NonExistent`}, Separator: '/', Mailbox: "a/b"})
 
 	tc.transactf("no", "delete a/b") // Already removed.
 	tc.transactf("ok", "delete a")   // Parent can now be removed.

imapserver/server.go

@@ -1237,7 +1237,12 @@ func (c *conn) applyChanges(changes []store.Change, initial bool) {
 				c.bwritelinef("* %d FETCH (UID %d FLAGS %s)", seq, ch.UID, flaglist(ch.Flags).pack(c))
 			}
 		case store.ChangeRemoveMailbox:
-			c.bwritelinef(`* LIST (\NonExistent) "/" %s`, astring(ch.Name).pack(c))
+			// Only announce \NonExistent to modern clients, otherwise they may ignore the
+			// unrecognized \NonExistent and interpret this as a newly created mailbox, while
+			// the goal was to remove it...
+			if c.enabled[capIMAP4rev2] {
+				c.bwritelinef(`* LIST (\NonExistent) "/" %s`, astring(ch.Name).pack(c))
+			}
 		case store.ChangeAddMailbox:
 			c.bwritelinef(`* LIST (%s) "/" %s`, strings.Join(ch.Flags, " "), astring(ch.Name).pack(c))
 		case store.ChangeRenameMailbox: