When I reply to an email, it is automatically marked as read. I want to mark it manually after I make sure there are no more actions required for this particular email. (Sometimes I want to answer one issue from an email and leave some more for answering.)
Outlook – How to turn off marking emails as read when replying in Outlook 2010
microsoft-outlook-2010
Best Answer
Unfortunately, Outlook 2010 does not seem to have that feature. Replying to a message marks it as read.
Though, if I may suggest an alternate approach to your problem: Consider the solution of having anything in your inbox being "unprocessed", and then using folders for what you have dealt with. Completed emails shouldn't be in your inbox view anyway. You are clearly a power-user of email, perhaps you could even get some peace of mind by not relying on Outlook marking emails bold and not?
To address your other concern: If you are in your inbox and hit the "From" button, you get a straightforward grouping of emails. That way you could easily go through "all the work that needs to be done for a given customer". You could also search, but only in your inbox, for the same effect.