As a constant user of Lotus Notes I have to say that one of the best features I have found in this tool is "Follow Up" which helps me a lot to don't lose the important mails I have to answer as soon as possible.
To remark one mail in this way, the first thing to do is to select it.
Then, you have to go to the "Follow Up" option (remember that the images are from a Spanish version) and click on "Add or Edit Flag...".
Suddenly, Lotus Notes will pop up the following window, where you can give this message a priority and actions, configure these and click "OK".
In this way, you will mark the massage with a priority represented by a flag icon together the mail.
But, if you want to see all your mails marked with some kind of priority you have to display the "Follow Up" section located at the bottom of the Lotus window.
Once you have expanded this section you will be able to see all your important mails, only the ones you have to review immediately.
However, doing the previous step I wrote above could sound very boring, it really is, this is why there's a faster way to do this: using the "Quick Flag" option, with this you will mark the message with a flag.
When you finish your work with the prioritized mail, you can remove the mark with the "Remove Flag" option.
As you can see, this is a powerful option when you have tons of mails there.