from outlook help: size limit
To send large messages

Many e-mail and news servers limit the size of the messages you can receive and send. Usually this limit is one megabyte (1 MB) per message, including all attached files.

With Outlook Express, you can send large messages or files to e-mail and news servers that have size limits, by breaking the messages into smaller ones. When the group of messages is received, the e-mail program combines them into one message.

On the Tools menu, click Accounts.
On either the Mail or News tab, click Properties.
On the Advanced tab, select the Break apart messages larger than x KB check box and then enter the maximum file size the server will allow.