Outlook 2000 has problems with big attac

My OL2000 (on Win98) is frozen when some of my emails has attachments with a size bigger than, let’s say, 600Kb.
I’ve contacted my ISP and they told me it’s a known problem with all Outlook versions.
I don’t want another email-client, but this is very annoying.
Does someone know a solution or a workaround?
Thanx!