Personally I don't like to use Outlook but if you have access to it you can have it automatically breakdown a file, but be aware that it will break it down into as many emails as it needs to cover the size of the attachment.
click the Outlook Express?s Tools menu and then click the Accounts option.
Click the Mail tab (assuming you?re sending email and then click Properties.
Click the Advanced tab, and select the check box that says Break apart messages larger than x KB. If you know the maximum size your recipient?s mail server can handle, enter that value. Otherwise, accept the default of 60.
What your recipient will see is a series of small files entering their mailbox. The good news is that their mail program will put Humpty Dumpty together again when all the little pieces have been received.
Hint: Before you do this, it might be a good idea to let the recipient know what?s about to descend on them, so they won?t panic.