How can I delete multiple or all inbox messages in the Microsoft Outlook Web Access 2003?
Did you try logging into your OWA with some other browser than Internet Explorer or Firefox ?
Opera for instance is not fully supported which can actually be a good thing since you don't get all that fancy Outlook imitation. The interface is much simpler and functionalities are much more available. You should then get a "check all" and be able to delete with increased speed.
Else, best solution would still probably to be able to configure and connect an Outlook client to your OWA...
If using OWA 2007 (I do not have 2003 installed to test) there should just be some checkboxes that you can tick to affect multiple items, then just click delete.
(OWA 2007 on Firefox)
If you are using OWA 2003 or 2007 in Internet Explorer, I believe that you can hold down shift to highlight multiple items in order or control to select multiple items that are separate to each other.
Shift-click does work — but only if you don't scroll too far.
I find I can get about two screenfuls of scrolling for each chunk added to the selection.
If you go too far by mistake, it's not a problem, just scroll back up a little closer and try again. You can scroll back to the top to verify the selection is still intact without losing what you've done. It's possible that zooming out (cmd--) helps, but only a little bit, even if you zoom a lot. (It seemed like I could pick up maybe 10 more items per click.)
None of the other solutions here or in a near-duplicate question worked for me, though my case was slightly different (selecting in order to move, rather than delete messages). I tried using Opera, Shift-click or Cmd-Shift-click to select the whole range at once, or most frustratingly Cmd-A.