Outlook Attachment Sending issue

I am having an issue when sending email to a web mail account. I can send a message with/without an attachment smaller than 1MB from my domain with Exchange 2003, but nothing larger they get rejected with an NDR. The web mail account is not blocking larger attachments because I am able to send from another web mail account with attachments tested to 5MB. I can also send large files from the web mail to the domain account. Messages are sent using either Outlook 2003 or 2007 accounts.

Thanks