Windows live mail at least will do that. It has each account listed seperately on the left and an "All inboxes" or something at the top so you can see everything at once.
System Manufacturer/Model Number Scratch built OS Windows 7 x64 Ultimate CPU i7 960 Motherboard Asus P6X58D Memory 12 Gig Corsair Dominator Graphics Card Nvidia 480 Sound Card Maudio Delta 44 + breakout box Monitor(s) Displays Dell UltraSharp U2410 24in and Samsung 21 dual monitors Screen Resolution 1920x1200 and 1280x1024
Keyboard Logitech G15 + N52 game pad Mouse Logitech MX518 PSU Corasair TX850 Case Cooler Master HAF Cooling Corsair H50 Hard Drives Primary: Intel X-25M G2 160G SSD
Secondary: Segate baracuda 1.0 TB
HDs in AHCI mode. Internet Speed 15kbs down 4.5kbps up Other Info WEI 7.6
CPU & RAM 7.6
Graphics 7.9
Hard disk 7.7
Thunderbird will do that to, you may have to check your individual e-mail service to see if it is allowed I have a Juno account that I can only check through a web browser it will not let me check it through Thunderbird.
Jerry
You may need to change your settings so that it doesn't do that.
As an example on the Gmail account I have you can change the settings so that it saves it or delete it.
That's through the web browser.
Jerry