Here’s a quick tip I ran into a while back. If you’re using Apple’s Mail app on an iPhone, you can mark a messages read by performing the following steps:

1. Start on the list of messages you want to mark read (e.g Inbox)
2. Tap the ‘Edit’ button
3. Select an unread message
4. Tap and keep your finger on the ‘Mark’ button
5. Deselect the selected message (while continuing to hold down the ‘Mark’ button)
6. Release the ‘Mark’ button and tap ‘As Read’
7. Mail will mark all of your messages as read

There is no way to undo this