Attachments problems

I have an Exchange 2003 server and my clients are using Outlook 2003. I am upgrading the Outlook 2000 clients to 2003. Once I do – they cannot send or recieve attachments, and I cannot find the setting to allow attachments. It also seems like when an Outlook 2003 client sends to someone using Outlook Express – the attachment just disappears with no errors or anything. The e-mail shows up in the reciepents mailbox with no attachment, no errors, no nithing – just the e-mail itself.

Help.