In the Mail application from Mac OS X Lion, is it possible to highlight somehow parts of the text of the emails? I want to store some mails and some weeks later, just read the most important parts (already highlighted) of those emails
You can distinguish messages that have important text you want to read later. You can do this by replying to the message, then saving the message as a draft, so you can come back whenever you want. (Copy the important text to your reply, then save it as a draft). When you want to look back at some of these important emails, you'll see the text snippets you think is important. If you want, I'll add instructions on how to highlight the emails, so you can find them easier.
(Here’s how to save a draft message as draft):
Choose Save As Draft from the File menu.
Your drafts are stored in the Drafts mailbox or the location you choose next to "Save unsent mail in" in the Composing pane of Mail Preferences.
To reopen a draft, open the mailbox where you store drafts and double-click the message.