Press F1 to get the help menu from here you can find how to send an e-mail that you suspect is to large. If you ask your friend to confirgure this on their computer it may fix the problem.
Extracted from Help.
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.