I use InfoSelect instead of Outlook to manage email. IS allows you to organize email under what it calls "Deliveries". A Delivery is often a name (email address). IS is also a free form random information organizer. Email can be organized any way you want, and therefore can work well with GTD.
When email comes in, it flows right into the correct "Delivery" and then gets as "task" icon attached to it (looks like an exclaimation point in a circle). You can see which parts of your outline structure have new email in them. My system is organized into catagories with the important ones near the top. Many emails that flow into low priority catagories can be left for my processing time later in the week.
Spam, by definition, does not fit any of my catagories so it always appears in the non-filtered inbox. The first 15 or so emails are normally spam. I just select them, delete them, and I'm done.
Likewise, email from people who are not part of my current projects (and therefore are not part of my organization system) end up in the unfiltered email inbox. A quick scan, and I either drag the entire Delivery into the correct portion of my system, or I simply answer and delete.
Part of the power of this is that the email that can wait until processing time is colapsed into my outline. I know it's there, but it's not right in my face. On the other hand, if I am anxiously waiting for an important reply on a current project, I can keep that section expanded, and instantly see the reply when it comes in.
When the time comes to process email, I move from section to section. Many emails are simply scanned and done. All I do is click the "task" icon and the email is no longer marked new. It is already filed, so I'm done.
More important email is dealt with either by dragging it to a task area, copying it to a task area (if I want to keep a copy elsewhere in the system) or deleted. If I want an archive, I drag it to an appropriate archive area.
Another huge advantage is that IS has a very fast and very good search engine. I hit F5, and start typing a person's name. A graphical search grid shows all 8000 pieces of data in my database. As I type each letter, the grid gets smaller until I feel it is small enough to easily find what I am looking for. I then hit Enter, and my entire outline is shrunk down to just what I am searching for. For example, if I type the name of a project, I will see all the email, project notes, calendar items, "maybe-somedays", etc. associated with that project.
I love GTD because I have lots of dissimilar projects to manage on any given day. InfoSelect allows me to set up the data my way, and then rearrange it later if my needs change. I like having email included in the same system, instead of in a seperate module like Outlook does.
My IS data base has over 8000 items in it, and I can find just want I need, using GTD, in seconds.