Is it possible to highlight text of inbox emails? In Preview.app, I can highlight and annotate text in a PDF file. Can we do the same thing to emails in my inbox in Mail.app?

I have long time collaborate projects and I need to go back to previous emails very frequently — it would be great if I can highlight texts in emails.
Apple Mail doesn't have a way to do this natively.  I can't think of another email client that does what you're describing either.  Here are some ideas and workarounds that might help you accomplish something similar to what you describe; perhaps one of them will suit your needs.

The closest option in Apple Mail is to use flags.  That only gives you the ability to flag a message.  You can't even add a note, let alone highlight the text and annotate it.  

You could try a third-party application like MailTags (Mavericks 10.9 support is currently in beta).  It's a paid application with a robust tagging system, as well as the ability to add notes.  This isn't a perfect solution since it doesn't allow you to highlight specific text within the email.  MailTags has a free demo; you might want to download it and see if you can reach a solution that works for you.

Another option would be to try Outlook:Mac.  It has both categories and flags built in.  You can also do things like create a task or a note from a message.  Once you have created a task or a note from a message, you can do things like highlight text, but not annotate it.  It also isn't the original message, but rather a copy of the message.  

If you do find something that works for what you want to accomplish, please do share it here.  Being more effective with email is a goal for many people, after all.