Question: I am accessing my colleague's work email account (on gmail) using IMAP. The problem is that whenever I read one of the messages first, it is marked as read on the server and he often misses it. I would like to keep it unmarked on the server and mark it as read only at my Mail app. 

Is there any way how to setup Mail app so that this behaves like this?

I am accessing his email because he often gets information that is important for me as well and this saves time because he isn't always online and he would have to forward most of it to me anyway.

Answer: 
There is a free program called True Preview that you can install to prevent Mail from automatically marking messages as "read". We use this all the time because we want messages to remain unread until such time as we actually mark them as "read". True preview has options to have different settings for different accounts, which is useful.

<http://christianserving.org/project/truepreview>
I had same issue, the culprit was the iPhone's email client with its default preview feature. A preview fools the IMAP to believe it's read. Solution: to switch it off (go to settings -> email -> set preview to none.