If you accidentally opened an email on your iPhone or iPad and don't currently have time to read it or respond, there are two things you can do to remind yourself to respond later. You could always flag it so you remember it needs attention later. If you'd prefer your Mail app to show a badge so you know something is there that hasn't been attended to, you can also mark it as unread.
Launch the Mail app from the Home screen of your iPhone or iPad.
Tap on All Inboxes.
Tap the Edit button in the upper right hand corner.
Find the mail message (or multiple messages) you'd like to mark as unread and tap on each one.
Once they're highlighted, tap on the Mark button in the lower right hand corner.
And lastly, tap on Mark As Unread.
That's all there is to it. The mail messages that you chose will now appear to be unread and the badge will re-appear on the Mail app just like they were never opened.