Mac OSX Mail; how to delay 'Mark as Read'
The Plugins MailHub and Mail Act-On have such options:

MailHub


  MailHub is a 'smart' plug-in for Apple Mail that revolutionises email management. It learns as you work, suggesting likely filing locations to take the pain out of keeping your mail organized. It also allows you to set reminders on emails, so you can follow-up later.


Mail Act-On


  Mail Act-On 3 new features give you unparalleled ability to manage your messages. Use Act-On's new templates to quickly reply to messages. Use Act-On composer options to specify when messages are delivered, where sent messages get archived, and even which outbox rules are applied.