There appears to be a bug in webmail when the in-box contains a message with a very large attachment. Attempting to open a message by clicking the subject link brings a 'message not found' error. Messages can be previewed OK by marking for selection and clicking 'view message'. Clearing the InBox by downloading brings everything back to normal.
By large attachment I mean probably in excess of 10 MB - it's been hard to pin down exactly. OK, e-mail shouldn't be used for attachments of this size. But in my job I receive a lot of unsolicited - but acceptable - mail which often carries multiple graphics files. Being unsolicited, FTP is not an option.
I was recently away from the office so relying on Webmail on a 'public' computer. After two days my InBox had become impossible to use. Its volume made it snail slow, and a few bulky messages triggered the bug which stopped me opening anything. The 'view message' preview is fine, but you can't delete from there. Web2Mail let me read messages but I couldn't delete the Spam, so everything ground to a halt.
I guess an option would be to create a 'bulky messages' folder and drag anything with a big attachment in there to clear later. But it would be good if the bug could be hunted down and shot.