Outlook express not deleting some emails off the server

[Mr. tech]

Some emails sent through to my client aren't getting deleted off the server. I think there might be a problem with her Outlook Express.

Because the emails wont delete, Outlook Express keeps trying to download them and then fails...

When the email that appears to be stuffing up is deleted, the emails come through normally again until another one clogs the server.

Is it Outlook Express? Is there a known fix for this? I've asked her to run a windows update...