How about having different colors on the tabs in Lotus Notes client?
The idea would function similar to how Colorfultabs work in Firefox: https://addons.mozilla.org/en-US/firefox/addon/colorfultabs/
Examples of implementation:
1) Different color for a) inbox b) mails under construction c) incoming mail d) calendar e) calendar entries f) databases
2) Just random colors to just easy separate tabs
3) Based on the senders email address. So if I open a mail where any of the persons is a certain person the tab color will reflect this. Maybe red color for some hot shot boss.
I vote for 1) but the other two is also ok.
As you see there is many ways to do this and increase efficiency when working with the mail application!