Outlook 2000 e-mail error!!!

When users try and e-mail attachments larger than 1 MB, an "Internal Application Error" pops up. I've checked their settings on the Exchange server (v.5.0) and everything seems to be in order. Rarely this will happen with attachments smaller than 1 MB, too, but not very often. Restarting Outlook tends to fix that problem. Any suggestions?

Thanks!