Greetings,
Without knowing the particulars on your mail server, or the limits that may be placed by the mail server, any answer is a "best guess". Many mail and news servers limit the size of the messages you can receive and send. This limit may be one megabyte (1 MB) per message, including all attached files, it may be more. Your mail server properties (or your ISP) could provide you with that information, and possible be changed accordingly.
If you can't get the mail server information,the following may be a "work-around".
With Outlook Express you can send large messages or files to mail and news servers that have size limits by breaking the messages into smaller ones. When the group of messages is received, the mail program recombines 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.
Good luck,
Maxwell