RFC 3501 states in section 6.1.2. that you should use the NOOP command for polling.
Though in TIdIMAP4 there's only the KeepAlive method using it, which is implemented as a procedure, i.e. doesn't return anything.
So how to check for status updates like e.g. new messages or read status changes? I.e. how can I do manual polling with TIdIMAP4? Which methods and properties are involved in doing that? And how to get the (U)IDs these messages?
Or is it even possible to use the IDLE command specified in RFC 2177 to avoid polling and to get updates automatically?
IMAP is technically an asynchronous protocol, but TIdIMAP4 is currently implemented as a synchronous client. As such, unexpected/out-of-order data is either discarded, treated as untagged data, or treated as error data, depending on timing and context. Untagged/extra data is accessible from the TIdIMAP4.LastCmdResult property, which you can type-cast to TIdReplyIMAP4 to access its Extra sub-property.
IDLE is not currently supported in TIdIMAP4. There are tickets in Indy's issue trackers (see here and here) to add IDLE support in a future release, maybe in Indy 11. Until then, you will have to poll the mailbox envelopes periodically, keeping track of messages you have already seen so you can detect new messages.