e-mail attachments won't open

I've had a problem opening certain e-mail attachments recently. These particular attachments are the ones represented by a white envelope icon. Whenever I click on the "paper clip" the white envelope icon appears whited- or greyed-out, meaning I can't open them. All other types of attachments appear to open properly.

I figure this is probably a rather common problem, but I've never had it before.

I would appreciate hearing from anyone who has a solution.

Thank you.

 

What email software brand- and version are you using?

 

I'm using Outlook Express 6.