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: 
The workaround is to change the number of seconds required for the mail to be marked as read. The default value is 2.

Please type the below command in terminal and it should do the trick. Tested on Big Sur (11.2.3)

```
defaults write com.apple.Mail MarkAsReadDelay 0

```
As other answerers have described, this is a feature of IMAP. IMAP maintains client-server synchronization so you will see the same thing regardless of whether you are using the browser, your desktop or your phone.

An alternative is to use [POP](https://support.google.com/mail/bin/answer.py?hl=en&answer=12103), which is a one-way download of messages. You will not benefit from any server-side filtering/labeling you have implemented in the Gmail UI, but your messages will have a unique read state on each device and will remain unread in the Gmail UI or in your colleague's email client. Please make sure that you properly configure POP inside of Gmail to [keep downloaded messages in the inbox](http://support.google.com/mail/bin/answer.py?hl=en&answer=13273) -- note that Gmail is unusual in that this setting is controlled inside of Gmail on the Forwarding and POP/IMAP tab of Settings, rather than in your email client.