Now if you're reading a conversation that had unread messages when you opened it, when you mark it as unread, Gmail will only mark as unread the messages in that conversation that were unread when you first opened the message.

Editions included:
Standard, Premier, Education and Partner Editions

Languages included:
All languages supported by Gmail

How to access what's new:
After selecting a message in Gmail, click 'More actions' then select 'Mark as unread'. Only the unread messages when the conversation was opened will be marked as unread.

For more information:
http://gmailblog.blogspot.com/2009/03/small-but-helpful-change-to-mark-as.html

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