I realize this post is a couple of years old, but it was a high-ranked hit in Google, so I thought I'd respond.
Personally I've been using IQTell and it will easily meet your functionality requirements. It was designed from the ground up with GTD in mind. If it's in GTD, then IQTell does it. (Projects, Actions, Waiting Fors - all linkable), integrates with all common email platforms including Exchange, Google, Yahoo!, generic IMAP, etc. Also integrates with the common calendaring solutions out there. Once you tie your email account(s) into IQTell, you can use one key to assign an email to a bucket like Actions, Projects, Waiting For, etc. Or you can just use the e-mail gateway - send an email to your customized iqtell address and it automatically goes to your desired bucket. Nice syncing via cloud - you can use web, iOS, or Android UIs. (Also, integrates nicely with Evernote - forgot to mention that.)
All in all it's pretty sweet - I've been very happy with it. The support staff has been super-responsive to bug reports and suggestions for improvements.