Thanks for the response. I use Thunderbird right now and I was using Postbox for a while. Email is from my web host, Lunarpages. Using WLM 2011 (Build 15.4.3538.0513), Windows 7 Premium Home Edition.
All programs use the exact same setup. What's more, in WLM, the Sent folder under each separate account shows the emails I've sent using my other applications, it's just the All Sent Items folder that doesn't.