How does the iPhone know when an iMessage has been read?
Read Receipts only work with iMessage, not SMS text messaging or MMS photo/video messaging.How do read receipts get sent? When you receive a message, if your device has read receipts turned on it sends the iMessage servers a minimal amount of data: that the message was in fact read on the recipient's end. This gets pushed to the sender and they see the receipt under their last sent and read iMessage in that conversation. When you send a message, if the recipient's device has read receipts turned on it will do the same process on their end and you'll see when they read your message.When do read receipts get sent? When you read an iMessage in your Messages app, a read receipt is sent to the sender if you have read receipts on; it will also send this message if you expand an incoming notification banner to read the entirety of a message or if you have Siri read the message to you. However, it will not mark as read if you view it in Notification Center or if you have message previews on and see it in the notification either in a banner/alert or on the lock screen.What about "Delivered" receipts? With both iMessage and SMS/MMS messaging on an iPhone, messages you send out will be marked "Delivered" once sending is complete. For SMS and MMS messages, this means your phone has sent it to a cell tower and the network has begun transmitting it; it has not necessarily been actually delivered to the device of the recipient. With iMessage, the "Delivered" receipt should only appear on your device once one or more of the recipient's iMessage-compatible devices with the service turned on receive the message; they may not see it on their screen yet, but their device is downloading the message from Apple's servers. (Delay can be noticeable with photo or video messages more than with text) Once delivered, your recipient still hasn't necessarily seen the message or even gotten the notification yet... whether you are using iMessage or traditional SMS/MMS messaging.
How does an imessage appear to the sender when I'm in airplane mode?
IMessage also works when connected to WiFi
Does "Delivered" in the iPhone Messages app mean that the message was received by the phone, or read by the user?
As other have already pointed out, "Delivered" simply means the message was received by a device. However, the catch is that this doesn't even necessarily mean the message was received by a device that's anywhere near the recipient, so don't assume that just because it says "Delivered" that the recipient is in any position to actually get the message.However, to add a bit of further detail, this doesn't necessarily mean it was received by a specific device – merely any of the macOS, iOS, or watchOS devices that a user may have set up for iMessage. These devices can be an iPhone, iPad, iPod touch, Mac, Apple Watch, or any combination of multiple devices that are using the same Apple ID for iMessages.This can be a common problem when a user has multiple devices, such as a Mac and an iPhone.... If the user leaves their Mac on at home, it will receive any iMessages sent to their account, and everything will be marked as "Delivered" even if the recipient's iPhone is actually OFF.So you could send somebody a message, see "Delivered" yet the person may not be in any position to receive the message due to being out of data coverage or having their iPhone off -- in other words, don't assume that just because you see "Delivered" that the message is immediately accessible to the recipient.
How can you recover older iMessages?
To recover older iMessages, there are three ways to complete the job: using iTunes, using iCloud, and using some professional iMessages recovery app. If you have backed up your iMessages to iCloud or iTunes you should be able to restore your iPhone old iMessages from the backup.Method 1. Recover Older iMessages with iTunesIf you are sure that the iTunes backup you made before contains the old iMessages, then this could be an efficient way to retrieve older iMessages. But this way will erase everything on your device before recovering, and then put all data from the backup to your device. So, you’d better make a backup of the current files on your device firstly.Open iTunes on your computer and connect your device to the computer with its USB cable.Select your device when it appears in iTunes. Then choose 'Restore backup' option.Choose the backup stores your old iMessages, and then click Restore and wait for the restore time to finish.Method 2. Recover Older iMessages with iCloudSimilar to iTunes way, restoring from iCloud will also erase your device, and then put all things from the iCloud backup to your device. If you are not quite sure that the iCloud backup contains your wanted iMessages, this way is not recommended.Go to Settings app on your device > General > Reset > Erase All Content and Settings. This will delete all media and data and reset all settings.After the erase finished, now you can start setting up your iPhone. (Language , country and keyboard)Follow the onscreen setup steps until you reach the Apps & Data screen, then tap Restore from iCloud Backup option.Sign in to iCloud with your Apple ID. Choose Backup and start to restore iMessages and everything to your device.Method 3. Recover Older iMessages with iMessages Recovery AppIf above two ways fail to help you retrieve your wanted iMessages, then the last way to get them back is to use some safe and professional iMessages recovery app, which can help you find out the older iMessages hidden on your iPhone or iPad, and get them back to your device.Hope my answer gives you a good reference and help you recover your older iMessages successfully. Good Luck.