For my personal e-mails, if I need to action but
CBA doing it immediately, I just mark as unread - then deal with it later.
For my e-mails at work, I have numerous folders that e-mails are filtered into during the day - I decide if I need to action, they are marked as unread then dragged into my "Pending" folder.
This way, when I do get time to action/respond to those e-mails, I action in the time order that they arrived.
When that is done, I drag them back into their original folder(s).
I'm not one of those who immediately stop what I'm doing to read a new e-mail (9 times out of 10 they're pointless e-mails that have nothing to do with me, I just happen to be in a distribution group).
I wait for a gap in my workflow before looking at new e-mails - it's more productive.