When you want to keep track of a project -- like who you communicated with, and what you talked about & decided -- but not all of your transactions on the project are email messages, you can use Outlook's Journal funtion to collect the info in one place. The Journal is an often-overlook feature of Outlook. It can create a timeline of transactions (phone calls, conversations, meetings, etc.) that can be linked to one of your contacts.
For details on how this might work for you, read more at this Productivity Portfolio 5-minute Tip.
[As mentioned in LifeHacker, via Jeff Gilderson-Duwe]